Methods and apparatus for spray forming, atomization and heat transfer

ABSTRACT

The present invention is directed to methods and apparatus that use electrostatic and/or electromagnetic fields to enhance the process of spray forming preforms or powders. The present invention also describes methods and apparatus for atomization and heat transfer with non-equilibrium plasmas. The present invention is also directed to articles, particularly for use in gas turbine engines, produced by the methods of the invention.

RELATED APPLICATIONS

This application claims priority to and is a divisional application ofU.S. patent application Ser. No. 09/882,248, filed Jun. 18, 2001, whichclaims priority to U.S. Provisional Application No. 60/212,122, filedJun. 16, 2000, which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to methods and apparatus that useelectrostatic and/or electromagnetic fields to enhance the process ofspray forming preforms or powders. The present invention also describesmethods and apparatus for heat transfer using non-equilibrium plasmasand for atomization.

BACKGROUND OF THE INVENTION

Spray forming is a process by which a stream of molten metal is atomizedby a gas stream impinging upon it. The resulting atomized droplets arethen directed to a target by the gas stream, or the resulting atomizeddroplets are cooled to form a powder. Producing powders by typical priorspray forming methods results in a yield loss of 10-15%, and much of theloss is associated with powder being trapped in various areas of theapparatus rather than being delivered to the collection vessel duringthe process. In producing solid workpieces, known as preforms, typicalprior spray forming methods result in a yield loss of 25-40%, and asignificant portion of the loss is usually caused by over-spray andparticles bouncing off the surface due to their angular impact relativeto the normal of the preform surface. Various methods have beendescribed to recover and reuse overspray powder, such as, for example,U.S. Pat. No. 5,649,993, but these are not wholly satisfactory.

Because many powders and preforms are susceptible to damage to theirchemical structure by air and oxygen, they are often produced in ashield gas environment of nitrogen or argon. The flow of shield gas,however, must be turned off to allow workers to enter the chamber forcleanup, changeover and maintenance. Thus, any powder or preformremaining in the chamber becomes contaminated and unusable when air andoxygen enter the spray forming apparatus after the flow of shield gas isturned off.

Previously, gas streams or jets have been used to direct the path of theparticles involved in the spray forming process. The gas streamstypically consist of argon or nitrogen as the means of directing theparticles, and heat is removed from the workpiece through conduction orconvection.

Current processes for making powder metal products, particularly inmaterials used for critical aerospace applications, use a conventionalgas atomizing process. In this process, high-pressure gas is directed ata molten metal stream to break it into smaller droplets. The dropletssolidify as powder. For critical applications, the resultant powder isthen blended with batches of powder from other small melts. The blend isscreened to a small mesh size (325 mesh), canned and consolidated byextrusion into product suitable for manufacture into an aircraftcomponent. This method of manufacture is not efficient because severalsmall melts are required for blending, melts are made in conventionalceramic lined furnaces and hence result in oxide contamination, severalpowder handling operations offer opportunity for contamination, and manysteps in the process make the production operation costly.

Heat transfer using non-equilibrium plasmas has heretofore been poorlyunderstood and often incorrectly or inefficiently applied. There is aneed in the art for methods and apparatus that improve the yield andquality of powders and preforms produced by spray forming. The presentinvention is directed to these, as well as other, important ends.

SUMMARY OF THE INVENTION

The present invention overcomes the limitations of the conventionalpowder process by permitting a significantly larger melt to bemanufactured to powder, thereby eliminating the blending steps. Theyalso are melted and atomized in a ceramicless system, thereby minimizingthe contamination from the furnace linings. They are atomized in vacuum,thereby eliminating the need for screening and handling. They can eitherbe containerized and sealed in a vacuum or rapidly solidified to form asolid preform in vacuum, thereby eliminating sources of handling andhence possible contamination. Finally, the present invention will haveconsiderably fewer handling steps than conventional powder making, andthus will be more cost effective.

In one embodiment, the present invention describes apparatus comprisingdispensing means, collecting means, and means for directing moltenparticles from the dispensing means to the collecting means comprisingan electrostatic field and/or an electromagnetic field. Optionally, theapparatus may further comprise atomization apparatus and/ornon-equilibrium heat transfer apparatus.

In another embodiment, the present invention describes spray formingmethods comprising directing molten particles from dispensing means tocollecting means by producing an electrostatic field and/orelectromagnetic field between the dispensing means and the collectingmeans. Optionally, the apparatus may further comprise atomizationapparatus and/or non-equilibrium heat transfer apparatus.

In another embodiment, the present invention is directed to apparatuscomprising a melt chamber that comprises at least one orifice; a meansfor expelling a molten material through the at least one orifice in themelt chamber; and a means for applying a rapid electrostatic charge tothe molten material. Preferably, the means for forcing the moltenmaterial through the at least one orifice in the melt chamber is amechanical or electromechanical actuator or a pressure means. In apreferred embodiment, the apparatus further comprises a means forcooling the molten particle. Preferably, the means for cooling themolten particle comprises a means for generating a non-equilibriumplasma.

In another embodiment, the present invention describes methods forforming particles comprising producing a first molten particle; andapplying a rapid electrostatic charge to the first molten particle,wherein the rapid electrostatic charge causes the first molten particleto form at least one smaller second particle. Preferably, the firstmolten particle is expelled through at least one orifice in the meltchamber via mechanical means or by a pressure means. In a preferredembodiment, the at least one smaller second molten particle is cooled,preferably by a non-equilibrium plasma.

In another embodiment, the present invention is directed to apparatusfor transferring heat between a heat-transfer device and a workpiececomprising the heat-transfer device, wherein the heat-transfer device iselectrically charged or held at a potential; the workpiece, wherein theworkpiece is mechanically separate from the heat-transfer device; andmeans for transferring heat between the workpiece and the heat-transferdevice comprising a means for generating a non-equilibrium plasma. Theheat-transfer device can be either a heat sink or a heat source.

In yet another embodiment, the present invention is directed to methodsof transferring heat between a heat-transfer device and a workpiececomprising producing a non-equilibrium plasma capable of transferringheat between the heat-transfer device and the workpiece, wherein theheat-transfer device is electrically charged or held at a potential, andwherein the heat-transfer device is mechanically separate from theworkpiece. The heat-transfer device can be either a heat sink or a heatsource.

Accordingly, in various embodiments, non-equilibrium plasmas areadvantageously employed to effect optimal heat transfer, and thenon-equilibrium plasma must act with a heat sink/source that has athermal conductivity capable of removing the desired quantity of heat.While two or more electrodes have been used in the past to produce aplasma in a region of high heat, such as a weld zone, so that the plasmawould serve to conduct heat outward from the weld zone, therebyincreasing the surface area for heat, embodiments of the presentinvention are directed to the discovery that a non-equilibrium plasmamay be used to introduce heat into a workpiece as well as from aworkpiece. It has further been surprisingly discovered that under thecorrect conditions a non-equilibrium plasma can be used to efficientlytransfer heat in a vacuum.

The novel methods of the present invention are particularly useful inpreparing any metal article, such as articles for gas turbine engines,including, for example, airfoils, blades, discs and blisks.

Accordingly, in one aspect, there is provided according to the presentinvention an apparatus comprising: a dispensing means; a collectingmeans; and a means for directing a molten particle from the dispensingmeans to the collecting means comprising at least one of anelectrostatic field or an electromagnetic field. In another aspect isprovided the apparatus described above, wherein the means for directingthe molten particles from the dispensing means to the collecting meanscomprises an electrostatic field or an electromagnetic field. Theapparatus may further comprise at least one magnetic coil, and may alsofurther comprise a means for charging the molten particles. In oneembodiment, the means for charging the molten particles may comprise athermionic emission source or a tribocharging device. The dispensingmeans of the apparatus may be a gas atomizer, and may further comprise ameans for transferring heat from the molten particles. The means fortransferring heat from the molten particles may comprise gas conductionand/or convection and/or a non-equilibrium plasma.

In another aspect, there is provided according to the present inventionan apparatus comprising: a dispensing means; a collecting means; and ameans for directing a molten particle from the dispensing means to thecollecting means comprising at least one of an electrostatic field or anelectromagnetic field, and further comprising a means for transferringheat from the collecting means. The means for transferring heat from thecollecting means may comprise a means for generating a non-equilibriumplasma. In a particular aspect, the means for transferring heat from themolten particles comprises a first heat sink, wherein the first heatsink is electrically charged or held at a potential; and a means fortransferring heat from the molten particles to the first heat sinkcomprising a means for generating a non-equilibrium plasma. Thenon-equilibrium plasma may be a glow discharge or a cold coronadischarge.

In another aspect, there is provided according to the present inventionan apparatus comprising: a dispensing means; a collecting means; and ameans for directing a molten particle from the dispensing means to thecollecting means comprising at least one of an electrostatic field or anelectromagnetic field, and further comprising a means for expelling themolten particle through at least one orifice in the dispensing means;and a means for applying a rapid electrostatic charge to the moltenmaterial. The means for expelling the molten particle through the atleast one orifice may comprise a mechanical or electromechanicalactuator. In one aspect, the means for expelling the molten particlethrough the at least one orifice may be a pressure means that produces apressure in the dispensing means that is greater than the pressure onthe outside of the dispensing means. The pressure means may causeinterrupted flow of the molten particle from the dispensing means. Therapid electrostatic charge may be an arc discharge or an electron beam.

In another aspect, the present invention provides for a spray formingmethod comprising directing molten particles from a dispensing means toa collecting means by producing at least one of an electrostatic fieldor an electromagnetic field between the dispensing means and thecollecting means. The electromagnetic field may be produced by, forexample, means comprising at least one magnetic coil. The methodaccording to this aspect of the invention may further comprise chargingthe molten particles. Charging the molten particles may be accomplished,for example, using a thermionic emission source or a tribochargingdevice. In one aspect, the dispensing means may be a gas atomizer.According to this aspect of the invention, the method may furthercomprise transferring heat from the molten particle. Transferring heatfrom the molten particles may be accomplished, for example, by gasconduction and/or convection and/or non-equilibrium plasma. In anotheraspect, the method of the invention further comprises producing a secondelectromagnetic field. According to the invention, the method mayfurther comprise transferring heat from the collecting means, which maybe by a non-equilibrium plasma.

In another aspect, the present invention provides for a spray formingmethod comprising directing molten particles from a dispensing means toa collecting means by producing at least one of an electrostatic fieldor an electromagnetic field between the dispensing means and thecollecting means, further comprising applying a rapid electrostaticcharge to the molten particle, wherein the rapid electrostatic chargecauses the molten particle to form at least one smaller molten particle.In a particular aspect, the rapid electrostatic charge may be an arcdischarge or an electron beam. In another aspect, the method of theinvention may further comprise transferring heat from the moltenparticle comprising producing a non-equilibrium plasma that transfersheat from the molten particle to a first heat sink, wherein the firstheat sink is electrically charged or held at a potential. Thenon-equilibrium plasma may be a glow discharge or a cold coronadischarge.

In another aspect, the invention is directed to an apparatus comprisinga melt chamber comprising at least one orifice; a means for forcing amolten material through the at least one orifice in the melt chamber;and a means for applying a rapid electrostatic charge to the moltenmaterial. The rapid electrostatic charge may be an arc discharge or enelectron beam. The apparatus of the invention may further comprise ameans for cooling the molten material. In a particular aspect, the meansfor cooling the molten material may comprise a first heat sink, whereinthe first heat sink is electrically charged or held at a potential; anda means for transferring heat from the molten material to the first heatsink comprising a means for generating a non-equilibrium plasma. Thenon-equilibrium plasma may be a glow discharge or a cold coronadischarge.

In another aspect, there is provided a method for atomizing a particlecomprising producing a first molten particle; applying a rapidelectrostatic charge to the first molten particle, wherein the rapidelectrostatic charge causes the first molten particle to form at leastone smaller second molten particle. According to the method of theinvention, the first molten particle may be produced by melting amaterial in a melt chamber, and expelling the first molten particlethrough at least one orifice in the melt chamber. The rapidelectrostatic charge may be an arc discharge or en electron beam. Themethod of the invention may further comprise cooling the second moltenparticle by producing a non-equilibrium plasma that transfers heat fromthe second molten particle to a first heat sink, wherein the first heatsink is electrically charged or held at a potential. The non-equilibriumplasma may be a glow discharge or a cold corona discharge.

In another aspect, the invention provides for an apparatus fortransferring heat between a first heat-transfer device and a workpiececomprising a first heat-transfer device, wherein the first heat-transferdevice is electrically charged or held at a potential, and wherein thefirst heat-transfer device is a heat sink or a heat source; a workpiece,wherein the workpiece is mechanically separate from the firstheat-transfer device; and means for transferring heat between theworkpiece and the first heat-transfer device comprising a means forgenerating a non-equilibrium plasma. The non-equilibrium plasma may be aglow discharge or a cold corona discharge. The apparatus of theinvention may further comprise an external means for generating ormaintaining the non-equilibrium plasma. The external means forgenerating or maintaining the non-equilibrium plasma may be a thermionicemission, an RF electromagnetic radiation, an electromagnetic radiation,a magnetic field or an electron beam. The first heat-transfer device ofthe apparatus of the invention may comprise a plurality of heat-transferdevices. In a particular aspect, the apparatus of the invention mayfurther comprise a second heat-transfer device that may be mechanicallyand electrically separate from the first heat-transfer device, whereinthe second heat-transfer device is a heat sink or a heat source, andwherein the potential between the first heat-transfer device and thesecond heat-transfer device produces a non-equilibrium plasma.

In another aspect is provided a method for transferring heat between afirst heat-transfer device and a workpiece comprising producing anon-equilibrium plasma that transfers heat between the firstheat-transfer device and the workpiece, wherein the first heat-transferdevice is electrically charged or held at a potential, wherein the firstheat-transfer device is mechanically separate from the workpiece, andwherein the first heat-transfer device is a heat sink or a heat source.The non-equilibrium plasma may be a glow discharge or a cold coronadischarge. The method may further comprise generating or maintaining thenon-equilibrium plasma via an external means. In an aspect, the externalmeans for generating or maintaining the non-equilibrium plasma comprisesa thermionic emission, an RF electromagnetic radiation, anelectromagnetic radiation, a magnetic field or an electron beam.

In another aspect, the invention provides for a preform produced by themethods of the invention. The preform of the invention may be a near netpreform. There is also provided an article of manufacture produced bythe method of the invention.

These and other aspects of the present invention will become moreapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of an apparatus of the present invention wherein anelectrostatic field directs and accelerates molten particles to apreform.

FIG. 2 is a view of an apparatus of the present invention wherein anelectrostatic field directs and accelerates particles to produce a nearnet shape preform, and a non-equilibrium plasma controls the temperatureof the molten particles.

FIG. 3 is a view of an apparatus of the present invention wherein anelectromagnetic field directs and accelerates molten particles to apreform, a first non-equilibrium plasma controls the temperature of themolten particles, and a second non-equilibrium plasma controls thetemperature of the preform.

FIG. 4 is a view of an apparatus of the present invention wherein anelectromagnetic field directs and accelerates molten particles tocontrol particle collisions and resultant particle growth, and anon-equilibrium plasma cools the molten particles to form a powder.

FIG. 5 is a graph showing deflection versus applied voltage for a moltentin particle, as described in Example 2.

FIG. 6 is a view of a non-equilibrium plasma heat transfer apparatuswherein the heat-transfer device and the electrode producing thenon-equilibrium plasma are a single element and a dielectric fluid isused to transfer heat from the heat-transfer device to a large thermalmass.

FIG. 7 is a view of a non-equilibrium plasma heat transfer apparatuswherein the heat-transfer device and the electrode producing thenon-equilibrium plasma are a single element and the heat-transfer deviceis coupled to a large thermal mass via a heat pipe.

FIG. 8 is a view of a non-equilibrium plasma heat transfer apparatusthat can be used to cool powders or small workpieces (e.g., moltenparticles or preforms) in a vacuum.

FIG. 9 is a view of a non-equilibrium plasma heat transfer apparatuswherein the heat-transfer device and the electrode producing thenon-equilibrium plasma are separate elements.

FIG. 10 is a view of an apparatus wherein a vacuum and pressure chamberserves as the dispensing means (e.g., melt chamber) for a moltenmaterial, pulsed pressure in the head space above the molten materialproduces molten particles through a plurality of nozzles at the base ofthe dispensing means, and rapid electrostatic charging is applied as themolten particles exit the nozzles to produce smaller molten particles.

FIG. 11 is a view of an apparatus wherein a flow control rod in adispensing means (e.g., melt chamber) is manipulated to produce moltenparticles, and rapid electrostatic charging is applied as the moltenparticles exit the nozzle to produce smaller molten particles.

FIG. 12 is a view of multiple electrostatically induced atomizationswherein a droplet is atomized to form a plurality of smaller dropletswhich are further atomized to form a plurality of still smallerdroplets. FIG. 14 is a photograph of the droplets shown in FIG. 12.

FIG. 13 is a graph showing how primary atomized droplets are notsensitive to high voltage levels or electrode gaps once a critical valueis reached for a particular geometry.

FIG. 14 is a photograph showing primary, secondary and tertiary dropletsproduced from the experiment in Example 5. FIG. 14 is a photograph ofthe droplets schematically drawn in FIG. 12.

FIG. 15 shows an apparatus used for liquid metal flow against thedirection of gravity. FIG. 15A is a schematic illustration of FIG. 15.

FIGS. 16 and 17 show drops and droplets collected from an exemplaryseries of experiments described in Example 4. For each figure, thelarger drops (upper portion of the figure) are those collected duringthe control experiments, and the smaller droplets (lower portion of thefigure) are those collected during experiments using an electrostaticfield according to the invention.

FIGS. 18-25 show various views of a section of CPVC pipe, placed in theassembly of the apparatus of the invention in such a way as to surroundthe extractor ring and its supporting arm, permitting substantiallyhigher potential differences between nozzle and extractor before arcingand voltage breakdown. FIGS. 18, 19 and 20 show consecutive frames ofthe atomization of liquid metal against gravity without any appliedmechanical force other than that due to the head of liquid in thereservoir. FIG. 18A is a schematic illustration of FIG. 18.

FIG. 26 shows twin electrode melting as the source for the molten metalfor electrostatic atomizing.

FIG. 27 shows electron beam melting as the source for the molten metalfor electrostatic atomizing in vacuum.

FIG. 28 shows electron beam cold hearth melting as the source for moltenmetal for electrostatic atomizing in vacuum.

FIG. 29 shows ESR/CIG melting as the source for the molten metal forelectrostatic atomizing in vacuum.

FIG. 30 shows the atomized powder being collected in the bottom of theatomizing chamber.

FIG. 31 shows electrostatically atomized powder being collected as asolid preform after the powder is cooled via a non-equilibrium plasma.

FIG. 32 shows electrostatically atomized powder being collected in acan, where the can is transferred into a smaller chamber withoutbreaking the vacuum. In the smaller chamber, the lid may welded to thecan prior to hot working to a final product.

FIG. 33 shows the production of a solid ingot in a mold from a powderproduced by electrostatic atomization.

FIG. 34 shows three stages of electrostatic atomizing using plasma andone stage of electrostatic steering of the atomized powder.

FIG. 35 is a schematic diagram of the experimental set-up described inExample 5 for heat transfer using non-equilibrium plasmas.

FIG. 36 is an enlarged schematic diagram showing the dimensions ofBlocks A and B described in FIG. 35.

FIG. 37 is a graph showing the temperature decay in air from Block Awith and without the non-equilibrium plasma in atmospheric pressure,where the gap between the blocks was 1.5 inches, and the voltage appliedfor the non-equilibrium plasmas was 51 keV, and Block A was in −vepotential.

FIG. 38 is a graph showing the temperature decay in air from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻¹ Torr,where the gap between the blocks was 1.5 inches, and the voltage appliedfor the non-equilibrium plasmas was 0.7 keV with a current maintained at20 mA, and Block A was in −ve potential.

FIG. 39 is a graph showing the temperature decay in air from Block Awith the non-equilibrium plasma (changing polarity of Block A) andwithout the non-equilibrium plasma at a pressure of 10⁻¹ Torr, where thegap between the blocks was 1.5 inches, and the voltage applied for thenon-equilibrium plasmas was 0.6 and 0.8 keV with a current maintained at20 mA.

FIG. 40 is a graph showing the temperature decay in air from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻¹ Torr,where the gap between the blocks was 4 inches, and the voltage appliedfor the non-equilibrium plasmas was about 0.7 keV with a currentmaintained at 20 mA, where Block A was at a −ve potential.

FIG. 41 is a graph showing the temperature decay in argon from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻¹ Torr,where the gap between the blocks was 4 inches, and the voltage appliedfor the non-equilibrium plasmas was 0.6 to 0.9 keV with a currentmaintained at 20 mA.

FIG. 42 is a graph showing the temperature decay in helium from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻¹ Torr,where the gap between the blocks was 4 inches, and the voltage appliedfor the non-equilibrium plasmas was 0.6 to 0.7 keV with a currentmaintained at 20 mA, where Block A was at a −ve potential.

FIG. 43 is a graph showing the temperature decay in air from Block A atvarious current with the non-equilibrium plasma at a pressure of 10⁻¹Torr, where the gap between the blocks was 4 inches, and the voltageapplied for the non-equilibrium plasmas was 0.5 to 1 keV with a currentat 10 mA, 15 mA or 20 mA, where Block A was at a −ve potential.

FIG. 44 is a graph showing the temperature decay in air from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻² Torr,where the gap between the blocks was 4 inches, and the voltage appliedfor the non-equilibrium plasmas was 1.2 to 1.6 keV with a current at 20mA, where Block A was at a −ve potential.

FIGS. 45A-B are graphs showing the argon control and plasma experimentaldata and numerical simulation results (p˜1E-1 Torr) for the modeling ofthe experimental data set presented in FIG. 39. Control andnon-equilibrium plasma curves are separated into two graphs to make thecurve fit presentation clearer. FIG. 45A shows argon without plasma andshows the experimental and model results. FIG. 45B shows argon in thepresence of a non-equilibrium plasma and shows the experimental andmodel results. The γ factor necessary to relate the two model curves isγ=10.5.

FIGS. 46A-B are schematic drawings of nozzle and extractor ring by aside view (FIG. 46A) and a view looking up through the extractor ringtoward the nozzle (FIG. 46B).

FIG. 47 shows the profiles of electric field pendent drops, where theelectric field increases from left to right.

FIG. 48 is a graph wherein the line with squares shows the limitingcharge according to the Rayleigh Criterion, and the line with circlesshows the calculated charge applied to a primary drop using measuredvoltage and the geometry of the drop. Though the graph shows that theprimary drop should have been atomized into 4 to 6 times, some chargemay have escaped to the environment or with the secondary droplets.

FIG. 49 is a schematic diagram of the experimental set-up described inExample 6 for heat transfer using non-equilibrium plasmas.

FIG. 50 is an enlarged schematic diagram showing the dimensions ofBlocks A and B described in FIG. 49.

FIG. 51 is a graph showing the temperature decay in air from Block Awith and without the non-equilibrium plasma in atmospheric pressure,where the gap between the blocks was 1.5 inches, and the voltage appliedfor the non-equilibrium plasmas was 51 keV, and Block A was in −vepotential.

FIG. 52 is a graph showing the temperature decay in air from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻¹ Torr,where the gap between the blocks was 1.5 inches, and the voltage appliedfor the non-equilibrium plasmas was 0.7 keV with a current maintained at20 mA, and Block A was in −ve potential.

FIG. 53 is a graph showing the temperature decay in air from Block Awith the non-equilibrium plasma (changing polarity of Block A) andwithout the non-equilibrium plasma at a pressure of 10⁻¹ Torr, where thegap between the blocks was 1.5 inches, and the voltage applied for thenon-equilibrium plasmas was 0.6 and 0.8 keV with a current maintained at20 mA.

FIG. 54 is a graph showing the temperature decay in air from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻¹ Torr,where the gap between the blocks was 4 inches, and the voltage appliedfor the non-equilibrium plasmas was about 0.7 keV with a currentmaintained at 20 mA, where Block A was at a −ve potential.

FIG. 55 is a graph showing the temperature decay in argon from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻¹ Torr,where the gap between the blocks was 4 inches, and the voltage appliedfor the non-equilibrium plasmas was 0.6 to 0.9 keV with a currentmaintained at 20 mA.

FIG. 56 is a graph showing the temperature decay in helium from Block Awith and without the non-equilibrium plasma at a pressure of 10⁻¹ Torr,where the gap between the blocks was 4 inches, and the voltage appliedfor the non-equilibrium plasmas was 0.6 to 0.7 keV with a currentmaintained at 20 mA, where Block A was at a −ve potential.

FIG. 57 is a graph showing the temperature decay in air block-A inplasma and without plasma at pressure 10⁻² Torr, gap between blocks was4″, and the voltage applied for plasma: 1:2 to 1.5 kev, current 20 mA.

FIG. 58 is a graph showing the results for the modeling of theexperimental data set presented in FIG. 53 relating to argon and withoutplasma.

FIG. 59 is a graph showing the results for the modeling of theexperimental data set presented in FIG. 53 relating to argon and withplasma.

FIG. 60 is a graph showing the comparison of with/without plasma for(Gamma)=10 in Example 7.

FIG. 61 is a graph showing how primary atomized droplets are notsensitive to high voltage levels or electrode gaps once a critical valueis reached for a particular geometry.

FIG. 62 is a photograph showing primary, secondary and tertiary dropletsproduced from the experiment in Example 8.

FIGS. 63-64 are tables illustrating two sets of experimental data forliquid wood's metal atomization.

FIGS. 65-70 and FIGS. 74-76 are pictures illustrating a piece of CPVDpipe placed in such a way as to surround the extractor ring and itssupporting arm.

FIGS. 71-72 are pictures illustrating drops and droplets collected inexample 8.

FIG. 73 is a picture illustrating an apparatus used for example 8.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods and apparatus for enhancing sprayforming for the production of solid workpieces, known as preforms, andfor powders. It has surprisingly been found that the amount or yield ofpowder collected during spray forming can be controlled to anunexpectedly high degree by using an electrostatic and/orelectromagnetic field to direct the trajectory of particles in the sprayforming process. Additionally, the speed and/or direction of theparticles can be controlled to produce a solid workpiece using anelectrostatic and/or electromagnetic field. Using an electrostaticand/or electromagnetic field, the particles can be directed to variousareas of the preform at various times during the spray forming processto produce shapes. Using an electrostatic and/or electromagnetic field,particle size and trajectory can be controlled to avoid particlecollisions, and the resulting growth in particle size that occurs whenparticles collide, or to cause particle collisions if larger particlesize is desired for any purpose. Using an electrostatic and/orelectromagnetic field, the particles can be directed to areas where heatcan be added or removed from the particles to control the macrostructureof the preform or powder being produced. The shape of the electrostaticand/or electromagnetic field can also be manipulated to produce near netshapes by directing where particles build up to form the preform atvarious times during the process. Spray forming using an electrostaticfield and/or an electromagnetic field can enhance the yield of theprocess as well as improve (and control) the density of the resultingpreform.

The present invention describes methods and apparatus usingelectrostatic fields and/or electromagnetic fields for selectivelycontrolling the yield, quality or density of solid workpieces (preforms)and powders produced by spray forming. Surprisingly, the methods andapparatus of the present invention have been unexpectedly found toprovide enhanced yields of 95-99%, and unexpectedly provide workpiecesthat have a density that is 11-14% greater than the density ofconventionally-formed workpieces.

Preferably, the methods and apparatus of the present invention comprisea source of molten particles; a means for collecting the moltenparticles; and a means for directing the molten particles from thesource of molten particles to the means for collecting the moltenparticles.

The molten particles can be metallic or non-metallic. The term“metallic” includes metals and alloys, including, for example, iron,cobalt, nickel, aluminum, hafnium, zinc, titanium, niobium, zirconium,tin, copper, tungsten, molybdenum, tantalum, magnesium, stainlesssteels, bronze, brass, lithium alloys and nickel/cobalt basedsuperalloys.

The source of molten particles may also be referred to herein as a“dispensing means.” The dispensing means can be any known in the artincluding, for example, a container, an atomizer, a grinder, or othermeans of producing and/or dispensing the molten particles. Thedispensing means is generally electrically insulated. Preferably, thedispensing means is a gas atomizing means. Any gas atomizing means knownin the art may be used as the dispensing means in the present invention.

The acceleration, speed and/or direction of the molten particles can bemanipulated and controlled by an electrostatic field and/or anelectromagnetic field. The term “electrostatic field” can refer to asingle electrostatic field or a plurality of electrostatic fields. Theterm “electromagnetic field” can refer to a single electromagnetic fieldor a plurality of electromagnetic fields.

The means for collecting the molten particles may be referred to hereinas a “collecting means.” Generally, the collecting means is electricallyinsulated. For spray forming powders, the collecting means can be ahopper or other container. The container may comprise a lid and amechanism for closing the lid. The collecting means may have a geometricshape, including, for example, a near net shape. Preferably, thedistance between the dispensing means and the collecting means is fromabout 10 cm to about 250 cm, more preferably, from about 20 cm to about100 cm, and even more preferably, from about 25 cm to about 75 cm.

The invention may further comprise means for charging the moltenparticles before and/or after they leave the dispensing means. The meansfor charging the molten particles may comprise, for example, athermionic emission source, a tribocharging device, or the like.

In one embodiment, an electrostatic field is produced between thedispensing means and the collecting means by connecting the collectingmeans to the positive or negative polarity of a high voltage DC powersupply and by grounding the dispensing means. Preferably a positivepolarity is used. Generally, the high voltage DC power supply is betweenabout 4 kV and about 250 kV; more preferably, between about 8 kV andabout 125 kV; and even more preferably between about 12 kV and about 100kV. The molten particles may be induction charged by the electric field.The induction charge causes the molten particles to move along theelectrostatic field lines, thereby controlling the speed and directionof the molten particles and directing the molten particles from thedispensing means to the collecting means.

In another embodiment, an electrostatic field is produced between thedispensing means and the collecting means by connecting the dispensingmeans to the positive or negative polarity of a high voltage DC powersupply and by grounding the collecting means. Preferably, a positivepolarity is used. By connecting the dispensing means to the positive ornegative polarity of a high voltage DC power supply, the moltenparticles become electrically charged. The electrostatic field causesthe electrically charged molten particles to move along theelectrostatic field lines, thereby controlling the speed and directionof the molten particles and directing the molten particles from thedispensing means to the collecting means.

The apparatus can further comprise a high voltage DC power supply andone or more electrodes that are placed between the dispensing means andthe collecting means to shape the electrostatic field between thedispensing means and the collecting means. The electrostatic field thendirects the molten particles to the collecting means.

The apparatus can also comprise a plurality of high voltage DC powersupplies each attached to one or more electrodes that are placed betweenthe dispensing means and the collecting means that change the shape ofthe electrostatic field between the dispensing means and the collectingmeans in a time dependant manner to direct the molten particles tospecific areas or points on the collecting means. This embodiment canproduce near net shapes.

In another embodiment, an electromagnetic field is produced between thedispensing means and the collecting means by placing a magnetic coilbetween the dispensing means and the collecting means. The magnetic coilis connected to a power supply. The molten particles leaving thedispensing means are directed by the electromagnetic field to thecollecting means. Preferably, the magnetic coil is capable of moving sothat it can direct the molten particles to specific areas or points onthe collecting means. The molten particles can be directed to produce,for example, near net shapes.

In another embodiment, a plurality of magnetic coils can be placedbetween the dispensing means and the collecting means. Theelectromagnetic fields that are produced by the plurality of magneticcoils, which are singly or multiply energized to different magneticfield intensities, direct the molten particles to specific areas orpoints on the collecting means. The molten particles can be directed toproduce, for example, near net shapes.

The embodiments of the invention presented in the following figures arefor purposes of illustration only, and are not intended to limit thescope of the invention or the appended claims.

In FIG. 1, a dispensing means 201 produces molten particles 202, and anelectrostatic field 203 is produced between the dispensing means 201 andthe collecting means 204. The electrostatic field 203 charges the moltenparticles 202, which then causes the molten particles 202 to acceleratetoward the collecting means 204. The acceleration causes the solidworkpiece (preform) 205 to build up on the collecting means 204 with aminimum of over-spray and bounce-off, thereby enhancing the yield of theprocess. The process can also enhance the density of the resulting solidworkpiece (preform) 205. As shown in FIG. 1, the electric field ispreferably intensified in the area where the molten particles 202 leavethe dispensing means 201. The inventors have unexpectedly discoveredthat the electrostatic field is most intense and compressed at the pointjust after the molten droplet leaves the nozzle. Surprisingly, pulling adroplet apart works for water, but does not work for liquid metal. Toatomize a liquid metal, the inventors have discovered compressing theliquid or molten metal droplets.

In FIG. 2, a dispensing means 201 produces charged molten particles 202,and an electrostatic field 203 is produced between the dispensing means201 and the shaped collecting means 206 to accelerate the charged moltenparticles 202 toward the shaped collecting means 206. The accelerationand directional control of the charged molten particles 202 enhances thedensity of the solid workpiece, and produces a near net shape solidworkpiece 207. Optionally, a non-equilibrium plasma 24 is created in thepath of the molten particles 202 between two heat sink electrodes 209which are connected to an outside thermal mass 210 by a dielectricliquid which flows through pipes 211 by the motive force provided bypumps 212. The arrangement between the heat sink electrodes 209 and theoutside thermal mass 210 allows heat to be removed from the moltenparticles 202. The non-equilibrium plasma 24 between the heat sinks 209is produced, for example, by means of an AC glow discharge or a coronadischarge. The non-equilibrium plasma 24 transfers heat from the moltenparticles 202 to the two heat sink electrodes 209 which transfer theheat to the outside thermal mass 210.

In FIG. 3, a dispensing means 201 produces charged molten particles 202,and an electromagnetic field 213 is produced by a magnetic coil 214which directs the molten particles 202 towards the collecting means 204.This directional control of the molten particles 202 can reduceover-spray, thereby enhancing the yield of the spray forming process.The invention can also enhance the density of the solid workpiece 205.Optionally, a non-equilibrium plasma 24 is created in the path of themolten particles 202 between two heat sink electrodes 209 which areconnected to an outside thermal mass 210 by a dielectric liquid thatflows through pipes 211 by the motive force provided by pumps 212. Thearrangement between the heat sink electrodes 209 and the outside thermalmass 210 allows heat to be removed from the molten particles 202. Thenon-equilibrium plasma 24 between the heat sink electrodes 209 isproduced, for example, by means of an AC glow discharge or a coronadischarge. The non-equilibrium plasma 24 transfers heat to the outsidethermal mass 210. The non-equilibrium plasma 24 extends from the heatsink electrodes 209 into the path of the molten particles 202 and to theelectrically grounded solid workpiece 205 and the collecting means 204.In this embodiment heat is transferred from the molten particles 202,the solid workpiece 205 and the collecting means 204 by thenon-equilibrium plasma 24 which allows heat to be transferred to theheat sink electrodes 209 which transfer the heat to the outside thermalmass 210.

In FIG. 4, a dispensing means 201 produces charged molten particles 202,and an electromagnetic field 213 produced by a magnetic coil 214 directsthe molten particles 202 to spread out, thereby reducing the probabilityof their collision, and hence the formation of larger molten particlesand larger powder particles. Optionally, a non-equilibrium plasma 24 iscreated in the path of the molten particles 202 between two heat sinkelectrodes 209 that are connected to the outside thermal mass 210 by adielectric fluid which flows through pipes 211 by the motive forceprovided by pumps 212. The arrangement between the heat sink electrodes209 and the outside thermal mass 210 allows heat to be removed from themolten particles 202. A second electromagnetic field 216 produced by amagnetic coil 217 directs the cooled powder 218 to facilitate collectionin the container 219 which is automatically closed by a mechanism 220that attaches a lid 221 The entire powder manufacturing process can becarried out in a full or partial vacuum to reduce or eliminatecontamination of the powder by chemical interaction with gases.

The present invention may also optionally comprise a heat sink placedbetween the dispensing means and the collecting means; a means fortransferring heat from the molten particles to the heat sink to controlthe temperature of the molten particles once they have been ejected fromthe dispensing means; and a means for removing heat from the collectingmeans. The apparatus can comprise a means for transferring heat from themolten particles to the heat sink to control the temperature of themolten particles once they have been ejected from the dispensing means.The means for transferring heat can be gas conduction and/or convection.In addition to or in place of gas conduction and/or convection, anothermeans for transferring heat can be a non-equilibrium plasma.

The present invention also provides non-equilibrium plasmas fortransferring heat between a heat-transfer device and a workpiece. Inpreferred embodiments, the non-equilibrium plasma is used for removingheat from molten particles after they are dispensed and/orelectrostatically atomized, but before they are collected either as asolid workpiece or as a powder.

A class of plasmas known as non-equilibrium (NE) plasmas is producedwhen the temperature of the electrons in the gas exceeds the temperatureof the neutral particles and large ions in the gas by at least 100%.Since the thermal conductivity of non-equilibrium plasmas depends on theelectron temperature, the non-equilibrium plasmas will exhibit a highthermal conductivity. Since the temperature of neutral particles andlarge ions, which account for more than 99.9% of the mass present, islow, the overall heat content of the non-equilibrium plasma is low.Non-equilibrium plasmas used for heat transfer can be generated undervery high and very low pressure conditions using gases which are inertor benign to the material(s) (e.g., the molten particles of the presentinvention) involved in the heat transfer. Thus, non-equilibrium plasmascan be used to add or remove heat from a workpiece without theundesirable mechanical, thermal, or chemical effects associated withplasmas in local thermal equilibrium.

The present invention also describes methods and apparatus for heattransfer between a heat-transfer device and a workpiece (e.g., moltenparticles and preforms) using non-equilibrium plasmas. Non-equilibriumplasmas eliminate the need for mechanical contact between the workpieceand the heat-transfer device. There are many applications in whichmechanical contact between the heat sink and the workpiece is notphysically possible without undesirable damage to or chemicalcontamination of the workpiece, including, for example, spray forming,casting and other processes which use molten or non-solid substratestates.

In a preferred embodiment of the present invention, heat transfer isaccomplished using non-equilibrium plasmas wherein the neutral and heavyions have a temperature less than about 1000 K, preferably less thanabout 800 K, and more preferably less than about 600 K. Sincenon-equilibrium plasmas are produced when the temperature of theelectrons exceeds the temperature of the neutral particles and largeions by at least 100%, the electrons preferably have a correspondingtemperature of at least about 100 K, more preferably in excess of about2000 K.

“Heat-transfer device,” as used herein, refers to a heat sink or a heatsource. “Heat source” refers to the object that is becoming colder,i.e., supplying the heat. “Heat sink” refers to the object that isbecoming warmer, i.e., accepting the heat. It will be appreciated thatthe same object can function as a heat source and as a heat sink,depending upon the temperature variation in the other object, e.g., theworkpiece, during the spray forming process. Accordingly, by means ofthe invention it is possible to closely control the cooling (andheating) rate of the workpiece as a whole, as well as individual partsor sub-parts of the workpiece, and thereby to control those propertiesof the workpiece or parts thereof which are known to be affected bycooling or heating rate.

In the present invention, the heat-transfer device or heat-transferdevice electrode can be electrically charged or held at a potential.“Heat sink electrode” refers to the electrical potential source and theheat sink when they are integrated into a single object. “Heat sourceelectrode” refers to the electrical potential source and the heat sourcewhen they are integrated into a single object. “Heat-transfer deviceelectrode” is used to refer to either a “heat sink electrode” or a “heatsource electrode.” “Being held at a potential” refers to a DC offsetvoltage upon which an AC waveform may be superimposed.

The distance between the heat-transfer device or heat-transfer deviceelectrode and the workpiece and the voltage applied to the heat-transferdevice or heat-transfer device electrode and/or the workpiece isselected to create a non-equilibrium plasma between the workpiece andthe heat-transfer device or heat-transfer device electrode to provideheat transfer. The non-equilibrium plasma is in contact with theheat-transfer device or heat-transfer device electrode and theworkpiece, while the workpiece is not in mechanical contact with theheat-transfer device or heat-transfer device electrode. Theheat-transfer device or heat-transfer device electrode and the workpiecemay be electrically connected, preferably through wires and a highvoltage power supply.

Preferably, the heat-transfer device and the electrode producing thenon-equilibrium plasma are a single element, e.g., “a heat-transferdevice electrode.” An alternative embodiment uses a charged electrode toproduce the non-equilibrium plasma and a mechanically separateheat-transfer device, which is grounded or charged to about half theopposite potential of the electrode producing the non-equilibriumplasma. For example, the electrode may have a voltage of about 25,000 toabout 150,000 volts to produce the non-equilibrium plasma, while theheat-transfer device has a voltage about half the voltage of theelectrode, such as from greater than about 0 to less than about 75,000volts. It will be appreciated in this regard that the minimum generallydesirable voltage of the heat transfer device will be that voltage whichis required to be applied to effect in the workpiece the desiredtemperature, and so may approach 0, while the maximum generallydesirable voltage will be about one-half that of the electrode.Preferably, the electrode and the heat-transfer device are notelectrically connected.

Generally, the workpiece is electrically grounded or held at a potentialopposite to the potential of the heat-transfer device or heat-transferdevice electrode by a high voltage power supply. An object held at theopposite potential is one with a positive DC voltage applied to it whenthe other electrode is negative or vice versa. Opposite potentials areused to create the field strength required to produce a plasma. Thedistance between the workpiece and the heat-transfer device orheat-transfer device electrode is from about 10 cm to about 250 cm, morepreferably, from about 20 cm to about 100 cm, and even more preferably,from about 25 cm to about 75 cm. Generally, the electrical potential orvoltage between the workpiece and the heat-transfer device orheat-transfer device electrode is from about 25,000 to about 150,000volts DC, or from about 25,000 to about 150,000 volts AC.

The electrical potential applied between the workpiece and theheat-transfer device or heat-transfer device electrode produces anon-equilibrium plasma having a desired thermal conductivity. Thenon-equilibrium plasma is preferably a glow discharge or a cold coronadischarge. Alternatively, radio frequency signals, microwave signals orradiation can be used to produce the non-equilibrium plasmas. Thethermal conductivity of the non-equilibrium plasma is generally about2-10 times greater than the thermal conductivity of helium, preferably,about 5-10 times greater, and more preferably, about 8-10 times greater,and may exceed 10 times greater.

The workpiece can be any workpiece known in the art, including metalsand non-metals. As used herein, “workpiece” refers to and includes asingle workpiece or a plurality of workpieces. Nonlimiting examples ofworkpieces according to the invention include powders and/or preformsproduced by spray forming. The workpiece can be a plurality ofworkpieces having an average diameter of about 0.1 to about 10 cm. Theworkpiece can be a material or a section or portion of a material thatrequires a high rate of cooling to control solidification, therebycontrolling grain structure and other metallurgical properties, such as,but not limited to, articles for gas turbine engines, including, forexample, airfoils, blades, discs and blisks. Preferably, the workpieceis a molten particle or preform, as described herein.

The workpiece can be stationary or can move or pass through thenon-equilibrium plasma. A dispensing means, as described herein, can beused to move or pass the workpiece through the non-equilibrium plasma.After the workpiece moves or passes through the non-equilibrium plasma,it can be captured or accumulated in any collecting means known in theart, as described herein.

The heat-transfer device or heat-transfer device electrode is connectedto a thermal mass which allows heat to be added or removed from theworkpiece by the non-equilibrium plasma. Heat can be transferred fromthe heat-transfer device to the thermal mass by any method known in theart. Preferably, the thermal mass will be a large thermal mass. A largethermal mass is one which can accept or donate a significant amount ofthermal energy with only a small change in temperature. Heat can betransferred from the heat-transfer device to the large thermal mass byheat transfer means including, for example, a dielectric fluid, a heatpipe, a thermally conductive metal, a thermally conductive ceramic andthe like. Dielectric fluids include, for example, silicon, mineral oiland the like. Conductive metals include, for example, copper, aluminum,brass, silver, gold and the like. Conductive ceramics include, forexample, mullites, steatites and other ceramic forms. For example, adielectric liquid can be circulated through the heat-transfer device orheat-transfer device electrode through pipes by a pump that is used tomove heat between the heat-transfer device or heat-transfer deviceelectrode and the large thermal mass to keep the temperature of theheat-transfer device or heat-transfer device electrode constant duringthe heat transfer process. In another embodiment, the heat-transferdevice or heat-transfer device electrode can comprise a heat pipe totransfer heat between the heat-transfer device or heat-transfer deviceelectrode and the large thermal mass to keep the temperature of theheat-transfer device or heat-transfer device electrode constant duringthe heat transfer process.

As used herein, the term “heat-transfer device” or “heat-transfer deviceelectrode” can include a single heat-transfer device or heat-transferdevice electrode or a plurality of heat-transfer devices orheat-transfer device electrodes that may or may not be mechanicallyand/or electrically separate. For example, a plurality of heat-transferdevices can be used, wherein each individual heat-transfer device iselectrically connected to a high voltage power supply, such that thepotential between the plurality of heat-transfer devices produces anon-equilibrium plasma. The electrode, in conjunction with the voltageapplied by the power supply and the field gradient within the geometry,produces the non-equilibrium plasma. When a plurality of heat-transferdevices is used, the distance between the individual heat-transferdevices can be any desired distance, such as about 1 to about 2,500 mm,preferably about 1 to about 1,500 mm, and the voltage between theindividual heat-transfer devices can be any desired voltage, such asabout 25,000 to about 150,000 volts DC or about 25,000 to about 150,000volts AC.

When a plurality of heat-transfer devices is used, some of theheat-transfer devices can produce a potential equal to about half thepotential that is being used to produce the non-equilibrium plasma, buthaving the opposite polarity. For example, if two heat-transfer devicesare used, the voltage applied to the first heat-transfer deviceproducing the non-equilibrium plasma can be AC, and the secondheat-transfer device can be connected to a separate high voltage powersupply that produces a potential equal to about half the potential thatis being used by the first heat-transfer device to produce thenon-equilibrium plasma, but having a negative or positive polarity. Inanother embodiment, if two heat-transfer devices are used, the voltageapplied to the first heat-transfer device producing the non-equilibriumplasma can be AC, and the second heat-transfer device can be connectedto a separate high voltage power supply to produce a potential equal toabout half the AC potential being used by the first heat-transfer deviceto produce the non-equilibrium plasma, but having a positive or negativeDC polarity. In still other embodiments, when two heat-transfer devicesare used, the voltage applied to the first heat-transfer deviceproducing the non-equilibrium plasma can be AC, and the secondheat-transfer device can be connected to a separate high voltage powersupply producing an AC potential equal to about half the potential thatis being used by the first heat-transfer device to produce thenon-equilibrium plasma, but being out of phase with the potential of thefirst heat-transfer device that is producing the non-equilibriumplasmas. Thus, for example, the phase difference between the ACpotential in the first heat-transfer device and the AC potential in thesecond heat-transfer device can be adjusted between about 1 degree andabout 180 degrees, and is preferably about 180 degrees. In theseembodiments, the voltages are preferably between about 5 kV and about 75kV, more preferably between about 10 kV and about 50 kV, most preferablybetween about 15 kV and about 25 kV. Although two heat-transfer deviceshave been exemplified, it will be appreciated by one skilled in the artthat these principles may readily be applied to more than twoheat-transfer devices in view of the teachings herein.

In some cases, a chamber can be used to enclose or contain theworkpiece, the dispensing means, the collecting means, the means fordirecting the molten particle, the heat-transfer device and theelectrode or the heat-transfer device electrode, and the non-equilibriumplasma. Such a chamber can be used to regulate the gas species presentand/or the pressure. For example, the chamber may be evacuated andcompletely or partially filled with an inert gas (e.g., argon ornitrogen), or vice versa, to achieve the desired final metallurgy, tocontrol the oxidation of other non-metal materials being processed,and/or to prevent undesired chemical reactions during the processing ofmaterials, such as oxidation and nitridation. In a preferred embodiment,the pressure in such an enclosed chamber is less than atmosphericpressure, preferably from about 0.1 to about 0.0001 torr, morepreferably from about 0.01 to about 0.001 torr.

In some cases, the voltage between the heat-transfer device electrodeand the workpiece may not be sufficient to initiate and/or maintain thenon-equilibrium plasma. In such cases, an external means for generatingand/or maintaining the non-equilibrium plasma can be used.Alternatively, an external means for generating and/or maintaining thenon-equilibrium plasma can be used instead of using the electrodesand/or heat-transfer device electrodes. The external means can maintainand/or elevate the temperature difference between the electrons and theneutral and heavy ions in the non-equilibrium plasma by supplying energyto the electrons. The external means can be any known in the art,including, for example, electron beams, thermionic emissions, RFelectromagnetic radiation, electromagnetic radiation in the range offrequencies from soft ultraviolet to hard x-rays, or magnetic fields.

The embodiments of the invention in FIGS. 6-9 are for purposes ofillustration only, and are not intended to limit the scope of theinvention or claims. Although FIGS. 6-9 refer to a heat sink, oneskilled in the art will appreciate from the teachings herein that theheat sink can be replaced with a heat source. In FIGS. 6-9, theworkpiece 101 is preferably a molten particle or preform, as describedherein.

In FIG. 6, the workpiece 101 is electrically grounded or held at apotential opposite to the potential of the heat sink or heat sinkelectrode 102 by a high voltage power supply 103 connected by wires 104.An electrical potential is applied between the workpiece 101 and theheat sink or heat sink electrode 102 to produce a non-equilibrium plasma24 having a desired thermal conductivity. In a preferred embodiment, adielectric liquid 106 is circulated through the heat sink or heat sinkelectrode 102 through pipes 107 by a pump 108 that moves heat betweenthe heat sink or heat sink electrode 102 and a large thermal mass 109 tokeep the temperature of the heat sink or heat sink electrode 102constant during the heat transfer process.

In FIG. 7, the workpiece 101 is electrically grounded or held at apotential opposite to the potential of the heat sink or heat sinkelectrode 102 by a high voltage power supply 103 connected by wires 104.An electrical potential is applied between the workpiece 101 and theheat sink or heat sink electrode 102 to produce a non-equilibrium plasma24 having the desired thermal conductivity. In a preferred embodiment,the heat sink electrode 102 comprises a heat pipe 110 which transfersheat between the heat source or sink electrode 102 and a large thermalmass 109 to keep the temperature of the heat sink or heat sink electrode102 constant during the heat transfer process.

In FIG. 8, an AC electrical potential is applied between a first andsecond heat sink or heat sink electrode 102 by a high voltage powersupply 103 connected by wires 104 to produce a non-equilibrium plasma 24through which the workpieces 101 are passed. The source of theworkpieces 101 is the dispensing means 111 which may comprise acontainer, atomizer, grinder or other means of producing or dispensingthe workpieces 101. A means for collecting the heated or cooledworkpieces 101 is provided by the hopper 112. The dispensing means 111is contained within a chamber 113 and a vacuum pump 114 connected to thechamber 113 by a pipe 107 which serves to reduce the pressure within thechamber 113. This pressure reduction within the chamber 113 is oftendesirable to reduce or eliminate contamination by unwanted gasses andalso serves to reduce the voltages required to produce thenon-equilibrium plasma 24. In this embodiment, a dielectric liquid 106is circulated through the heat sink or heat sink electrode 102 andthrough pipes 107 by pumps 108 that move heat between the heat sink orheat sink electrode 102 and a large thermal mass 109 to keep thetemperature of the heat sinks or heat sink electrodes 102 constantduring the heat transfer process. In this embodiment, a plurality ofelectrically charged heat sinks or heat sink electrodes may also be usedand they may be oriented perpendicular to the direction of movement ofthe workpiece.

In FIG. 9, the workpiece 101 is electrically grounded or held at apotential opposite to the potential of the electrode 115 by a highvoltage power supply 103 connected by wires 104. An electrical DCpotential is applied between the workpiece 101 and the electrode 115 toproduce a non-equilibrium plasma 24 having the desired thermalconductivity, and which impinges on the surfaces of the workpiece 101,the electrode 115 producing the non-equilibrium plasma 24 and the heatsink or heat sink electrode 102. In this embodiment, a dielectric liquid106 is circulated through the heat sink or heat sink electrode 102 andthrough pipes 107 by a pump 108 that is used to move heat between theheat sink or heat sink electrode 102 and a large thermal mass 109 tokeep the temperature of the heat source or sink electrode 102 constantduring the heat transfer process. In this embodiment, the heat sink orheat sink electrode 102 is either grounded or held at a potentialopposite to that of electrode 115 producing the non-equilibrium plasma24 and having approximately 50% of the potential applied to theelectrode 115. The potential of the heat sink or heat sink electrode 102is controlled by a high voltage power supply 103 which is connected tothe heat sink or heat sink electrode 102 by a wire 104. In this case,the electrode 115 producing the non-equilibrium plasma 24 and the heatsink or heat sink electrode 102, which adds or removes heat, are twoseparate elements. Typically voltages in the range of 25,000 to 150,000volts are applied to electrode 115 to produce the non-equilibrium plasma24, while the potential of the heat sink or heat sink electrode 102 hasa voltage about half the voltage of electrode 115, such as from greaterthan about 0 to less than about 75,000 volts. The minimum generallydesirable voltage of the heat sink or heat sink electrode 102 will bethat voltage which is required to be applied to effect in the workpiecethe desired temperature, and so may approach 0, while the maximumgenerally desirable voltage will be about one-half that of electrode115.

Heat transfer using non-equilibrium plasmas has a wide range ofapplications, including, for example, arc welding, Mig welding, Tigwelding, laser welding, metal spraying of preforms and powders, powdermanufacture, and other metal fabrication and manufacturing processeswhich require a high rate of cooling, such as solidification and grainstructure control in the cooling of alloys, superalloy casts and welds.A surprising and unexpected aspect of the present invention, then, isthe use of electron flow within a non-equilibrium plasma to transferheat, which in aspects of the invention may be accomplished in a vacuum.

Preferably, the dispensing means of the present invention is anatomizing means. Atomization of molten particles using rapidelectrostatic charging results in the rapid breakup of particles intosmaller particles due to electrostatic repulsion forces. The productionof small particles has a wide range of commercial and industrialapplications, including, for example, powder production, spray formingand metal coating processes.

Advantages of the present atomization methods and apparatus overconventional gas atomization include, for example, that the presentinvention can be carried out in a vacuum so that chemical interactionswith the molten material can be controlled or eliminated, and any voidsin the solid workpiece (e.g., preform) produced by the present inventionwould collapse during subsequent working of the workpiece (e.g.,preform) so that no defects would exist in the final product.

In one embodiment of the present invention, a high voltage DC powersupply is used to rapidly electrostatically charge molten particlesbeyond the Rayleigh limit, such that the electrostatic forces within theparticles exceed the surface tension of the material and the particlesbreak up into smaller particles. The “Rayleigh limit” is the maximumcharge a droplet can sustain before the electrostatic repulsion forcesovercome the surface tension. This rapid electrostatic charge can alsobe used to further break up the particles resulting from the first rapidelectrostatic charge. Thus, several size refinements using rapidelectrostatic charging are possible. Preferably, electrostatic chargingis applied one, two, three, four or more times to refine the particlesto a desired size. The final size to which droplets can be atomized isbased on the applied voltage, the starting diameter of the particle, therate of charging of the particle, and the geometry of the electrostaticor electromagnetic field present.

In exemplary processes of the present invention, a material is placed ina container and liquified. The material can be metallic or non-metallic.The container can have one or more nozzles or orifices through which themolten material can flow. The container may also be referred to hereinas a “dispensing means” or “melt chamber.” The inside diameter of theorifice is preferably about 0.1 mm to about 10 mm, more preferably,about 0.15 mm to about 2 mm, yet more preferably, about 0.15 mm to about0.3 mm, most preferably, about 0.15 mm. When the inside diameter of theorifice is less than about 0.1 mm, it is difficult to achieve aconsistent flow of the liquid metal The size of the primary droplet(s)need not be minimized since the goal of the invention is not to achievea liquid metal spray at the tip of the nozzle.

In one embodiment, the dispensing means is sealed so that a vacuumand/or pressure can be created. The molten material is forced orexpelled through the orifice(s) by a positive pressure that is createdin the head space above the molten material. The pressure in the headspace can be increased and decreased (e.g., pulsed or oscillated) in atime dependant manner to cause molten particles to be formed at theorifice(s) due to the periodic interruption of flow of the moltenmaterial. When the particles are ejected from the orifice(s), they enterthe particle formation and collection chamber. The particle formationand collection chamber is preferably sealed so that a vacuum or pressurecan be created in the chamber and so that gases cannot contaminate themolten particles or final product.

The pressure in the head space above the molten material in thedispensing means is preferably equal to or less than pressure in theparticle formation and collection chamber to prevent molten materialfrom discharging from the orifice. The pressure in the head space abovethe molten material in the dispensing means is preferably increased byabout 1 to about 1,500 mm of mercury at a frequency of about 1 to about500 Hz, more preferably about 2 to about 200 Hz to cause interruptedflow (e.g., pulsed flow or oscillated flow) of the molten materialthrough the orifice(s). Any method of interrupting flow by, for example,creating a positive or negative pressure differential between the headspace and the dispensing means, or by electrical or mechanical means,may be used. This interrupted flow causes the molten particles to form.The molten particles formed at this point may be referred to herein as“primary molten particles” because they are the first particles formedin the process.

The primary molten particles can be charged in several ways. The moltenparticles can be rapidly charged by conduction charging in theorifice(s) (e.g., before being expelled from the orifice) or by anelectrostatic discharge into the molten particles as the moltenparticles are expelled from the orifice(s), and/or after the moltenparticles are expelled from the orifice(s). Preferably, the primarymolten particles are rapidly electrostatically charged. The rapidelectrostatic charge can be created by, for example, an arc discharge oran electron beam. As used herein, “rapid” is from about 1 to about 500microseconds, preferably about 1 to about 100 microseconds, mostpreferably about 1 to about 50 microseconds. The rapid charging of theprimary molten particles creates a plurality of secondary moltenparticles that have a uniform diameter of about 5 to about 2,500microns, preferably about 5 to about 250 microns. The secondary moltenparticles can be used to produce solid preforms or powders, or to coat asubstrate(s), as described herein.

In an alternative embodiment, a nozzle and a dispensing means arearranged so that a flow control rod is moved by a mechanical orelectromechanical actuator to allow the molten material to flow out ofthe nozzle through an orifice(s). Preferably, the flow control rod ismoved vertically by the mechanical or electromechanical actuator.Optionally, pressure or a vacuum can be applied in the dispensing means.The container can comprise one or a plurality of nozzles and flowcontrol rods. A high voltage power supply, capable of providing avoltage rise rate of at least 3 million volts per second, is connectedto the nozzle by a conductor. Preferably, the voltage rise rate is about100 to about 100 million volts/second, more preferably from about 500 kVto about 50 million volts/second, even more preferably from about 1million to about 30 million volts/second. The rise rate is the slope ofthe waveform where the x axis is time and the y axis is voltage. Thehigh voltage is applied to the nozzle at a high rise rate by the powersupply and conductor and is synchronized with the momentary retractionof the flow control rod by the mechanical or electromechanical actuatorwhich causes a primary molten particle to form. The high voltage appliedat a high rise rate causes the rapid electrostatic charging of theprimary molten particle which causes the primary molten particle tobreak up or atomize into smaller secondary molten particles due toelectrostatic forces.

The embodiment in FIG. 10 describes an apparatus for producing smallmolten particles that can be collected as a solid or used to coat asubstrate. The apparatus comprises a vacuum and pressure vessel 1 whichserves as the dispensing means. A vacuum source 2 is connected by a pipe4 a to a valve 3 a which is in turn connected to the vacuum and pressurevessel 1 by a tube 5 a. A pressure source 6 is connected by a pipe 4 bto a valve 3 b which is in turn connected to vacuum and pressure vessel1 by a tube 5 b. A computer 7 reads the temperature of the moltenmaterial 8 by a temperature sensor 9 which is connected to the computer7 by wire 10 a. The computer 7 reads the pressure in the vacuum andpressure vessel 1 by a pressure sensor 11 a which is connected to thecomputer by wire 10 b. The induction heat sources 12 are connected tothe computer 7 by wires 10 c. The positive side of the high voltagepower supply 13 is connected to an electrode 14 a by an insulated wire10 d and the negative side of the high voltage power supply 13 isconnected to electrodes 14 b by an insulated wire 10 e which passesthrough a vacuum tight insulated connector 18 and wire 10 f. A secondvacuum source 21 is connected by a pipe 4 c to a valve 3 c which isconnected to the particle formation chamber 20 by a tube 5 c and isconnected to the computer 7 by wire 10 g. A pressure sensor 11 b isconnected to the computer 7 by wire 10 h. The high voltage power supply13 is connected to the computer for control by wire 10 i.

In use, when the system initially starts, the computer 7 senses thepressure in vacuum and pressure vessel 1 and in the particle formationchamber 20 by the pressure sensors 11 a and 11 b, respectively. Thecomputer 7 then controls the evacuation of the particle formationchamber 20 by the valve 3 c controlling the second vacuum source 21 toproduce a pre-set partial pressure level specific to the material to beatomized, and controls the first vacuum source 2 and the pressure source6 by valves 3 a and 3 b, respectively, to maintain a partial pressure inthe vacuum and pressure vessel 1 equal to that in the particle formationchamber 20. The pressure is varied from atmospheric pressure down to alower pressure until the desired flow rate and resulting particle sizeis achieved.

The computer 7 then senses the temperature of the material 8 by thetemperature sensor 9 and provides power to the induction heaters 12 bywires 10 c until the material achieves the desired pre-set melttemperature which causes the material to liquefy. At this point, anormal atomization cycle begins.

Once the computer 7 senses that the pre-set melt temperature has beenreached, a positive pressure burst is applied to the vacuum and pressurevessel 1 by the computer 7 opening the valve 3 b to the pressure source6 thereby forcing some of the molten material 8 through the orifices 15to form the primary molten particles 16. The computer 7 then closes thevalve 3 b and momentarily opens valve 3 c and/or valve 3 a to equalizethe pressure between the vacuum and pressure vessel 1 and the particleformation chamber 20 which stops the molten material 8 from flowing. Thehigh voltage power supply 13 is then turned on by the computer 7 and arapid charging of the primary molten material particles 16 by theelectrical arcs 19 causes the electrostatic forces within the primarymolten particles 16 to exceed the surface tension energy resulting inthe formation of smaller secondary molten particles 17.

The secondary molten particles 17 will then pass through anon-equilibrium plasma 24 created by second electrodes 25 which eachtransfer the heat to the outside of the particle formation chamber 20 toheat exchangers 26. The resulting cooled atomized particles are thencollected by the collecting means 22 either as a solid preform or aspowder, depending on the amount of cooling provided by thenon-equilibrium plasma 24.

The cycle will then begin again at the point where normal atomizationbegins. Throughout the process, the computer 7 senses the temperature ofthe material 8 by the temperature sensor 9 and provides power to theinduction heater 12 by wires 10 c to maintain the desired pre-set melttemperature to maintain the material as a liquid. At the end of theatomization cycle, the computer 7, via a wire 10 j, opens a vent 23which is connected to the particle formation chamber 20 by a pipe 4 dand is connected to the air outside the particle formation chamber 20,which causes the pressure within the particle formation chamber 20 toequalize with the outside air pressure. Thereafter, the particleformation chamber 20 can be opened to remove the product.

In FIG. 11, a nozzle 30 and a dispensing means 1 are arranged so that aflow control rod 27 is moved by a mechanical or electromechanicalactuator 28 to allow the molten material 8 to flow out of the nozzle 30through an orifice 15. A high voltage power supply 13, capable ofproviding a high voltage rise rate, is connected to nozzle 30 by aconductor 31. The high voltage is applied to the nozzle 30 at a highrise rate by the power supply 13 and conductor 31 and is synchronizedwith the momentary retraction of the flow control rod 27 by themechanical or electromechanical actuator 28 which causes a primarymolten particle 16 to form. The high voltage applied at a high rise ratecauses the rapid electrostatic charging of the primary molten particle16 which causes the primary molten particle 16 to break up or atomizeinto smaller secondary molten particles 17 due to electrostatic forces.

In FIG. 12, a nozzle 30 and dispensing means 1 are arranged so that theprimary molten particle 16 exits the orifice 15. Thereafter, theelectrode 14 a releases an electrical arc 19 that causes theelectrostatic forces within the primary molten particle 16 to exceed thesurface tension energy, resulting in the formation of smaller secondarymolten particles 17. Subsequently, another electrode 14 b releases anelectrical arc 19 that causes the electrostatic forces within thesecondary molten particles 17 to exceed the surface tension energy,resulting in the formation of smaller tertiary molten particles 40.Thereafter, another electrode 14 c releases an electrical arc 19 thatcauses the electrostatic forces within the tertiary molten particles 40to exceed the surface tension energy, resulting in the formation ofsmaller quaternary molten particles 41.

The electrodes 14 a, 14 b, 14 c are rings of varying diameters,according to the electric potentials applied. Generally, they havediameters of about 1 to about 20 centimeters, preferably about 5 toabout 15 centimeters. The electrodes 14 a, 14 b, 14 c can be extractor,expansion or compression rings, preferably they are an expansion orcompression ring. An expansion ring is generally a bare metal wire ringthat is at an electric potential such that an attractive or expansiveforce is exerted on the charged droplet(s). A compression ring isgenerally a metal wire ring coated with a dielectric material of varyingthickness. When an electric potential is applied to the compressionring, an opposite charge is induced upon the surface of the dielectricmaterial, forming a squeezing (or compressive) force upon thedroplet(s).

Preferably, an extractor ring 80 is also used in the apparatus of thepresent invention. The extractor ring 80 is generally the ring closestto the nozzle 15 that encourages extraction of the primary drop 16 fromthe nozzle 15.

According to the invention, the atomization process is manipulated usingthe methods and apparatus of the invention to effect the production ofsmaller droplets. It has been found that the extractor ring, when usedin accordance with the invention as described herein plays a significantrole in controlling and/or maximizing the division process. While notintending to be bound by any particular theory, the wire rings seem topermit some expansive (sucking) force to be applied upon the droplet asit passes the ring plane, while PVC rings seem to permit a compressiveforce to be applied upon the droplets. The importance of maintaining theenvironment in the vicinity of the electrostatic field at a temperatureabove the melting point of the liquid metal cannot be over-emphasized.As the droplets become smaller their surface area to mass ratioincreases and they cool more rapidly.

Referring to FIGS. 46A-B, the distance between the nozzle 803 and theextractor ring 804 is generally about 1 to about 50 millimeters. Thediameter of the extractor ring 804 varies according to the voltages thatare applied. The extractor ring 804 will be at an electric potentialless than the nozzle 803 to cause liquid to be pulled from or extractedfrom the nozzle 803.

A positive high voltage DC source connected to the liquid metalreservoir produces an electric field between the nozzle and the groundedcollector cup. The force of the field produced, acting together withgravity, causes atomized droplets of similar size to be collected. Thisphenomenon is called primary atomization.

The placement of an extractor ring between the nozzle and collector cupand concentric to the droplet path causes lateral forces to be appliedto the droplet, which can produce successively smaller droplets. Thisphenomenon is called secondary and tertiary atomization, as shown inFIGS. 12 and 14. It is preferred to maximize the number of tertiarydroplets produced. FIG. 13 shows the weight of the droplets producedversus the gap between the nozzle and the extractor. This figure clearlyillustrates that once a critical value is reached, primary atomizationis not sensitive to the high voltage potential applied, or to thedistance between the nozzle and the extractor. Table 2 and Table 3 showthe results of experiments using bare copper wire extractors withdifferent ring diameters.

FIG. 14 shows evidence abstracted from the same experimental sample. Alldroplets were produced by the same experiment. The choice of solidifieddroplets P₀ to P₄ demonstrates the way in which the primary droplets aresubdivided. The choice of solidified droplets S₀ to S₄ shows thisphenomenon then repeats upon the secondary droplets to produce tertiarydroplets such as solidified droplet T_(o). Droplets P₄ and S₄ appearsevered, but they are whole, and droplet T₀ seems large for its weight.Unfortunately, these apparent anomalies arise from lens distortion dueto the scanning and copying processes involved in producing FIG. 14.

While not intending to be bound by any particular theory, the divisionof an initial liquid metal drop into smaller droplets seems to be theresult of three separate processes. Consider a case in which a liquidmetal drop is emitted in the direction of gravity from the capillarynozzle of a positively-charged reservoir towards a grounded baseplate.Assume that the surrounding environment is sufficiently warm for thedrop (or atomized droplets) to remain liquid. The size of the dropemitted depends directly upon the electric field applied to thereservoir. The drop will form at the nozzle in a manner shown in FIG.47, the size of the drop being governed by the field applied up to acritical value. Thereafter, only the rate at which the drop leaves thecapillary nozzle is affected by the field.

The possibility of liquid metal spray occurring directly from the nozzletip can be eliminated because the aperture of the nozzle (preferablyabout 0.15 mm inside diameter) is too large to permit formation of aTaylor-cone upon the free surface. The basic phenomenon of a liquid filmdeformation is that the outward electric stress (σ_(E)) has to overcomethe stress (σ_(S)) due to surface tension (i.e., σ_(E)≧σ_(S)). A chargeddrop begins to atomize when the applied force is in excess of theRayleigh limit q_(r)=8π(ε₀.T.r³)^(1/2) whereε₀, T, and r arepermissivity of free space, surface tension of a liquid, and dropradius, respectively. The downward force of gravity combines with theelectrostatic force to cause the drop to be ejected from the nozzlebefore sufficient charge can accumulate to constitute a force that canovercome the liquid metal surface tension forces. Liquid metals havehigh inter-molecular binding energies and thus have high values ofsurface tension so that drops are not readily torn off from the apex,even at reasonable field strengths. Hence, sufficient charge can neverbe created upon the liquid metal surface at the nozzle aperture by a DCfield to satisfy the Rayleigh Criterion.

Now consider a case in which the liquid metal drop is emitted from thereservoir in a manner similar to that described in the paragraph above,but in this case the drop passes through an extractor ring connected toground, or to a negative potential with respect to the droplet collectorcup, and is positioned concentric to the nozzle center and slightlybelow the nozzle tip (e.g., within about 1 to about 2 centimeters). Asthe drop falls toward a position that is coplanar with the ring plane,the field intensity between the drop and the ring increases.

If the potential difference between the drop and the ring is largeenough to impart sufficient charge upon the drop, then the RayleighLimit may be reached. As the drop continues to fall toward the ringplane, the Rayleigh limit is surpassed and highly charged particles withsmall mass are ejected, since the electrostatic forces have exceeded thesurface tension forces. These particles should be ejected at the surfaceof the drop where the electrostatic field is densest, and it can beestimated that up to 25% of the charge is carried away by the particles,leaving the majority of the mass of the drop remaining with a lessercharge. Since the charge remaining on this residual drop is less thanthat required to satisfy the Rayleigh Criterion, no further atomizationwill occur unless the drop is recharged by induction along its flightpath (see FIG. 48).

Finally, a case similar to that described in the paragraph above can beconsidered, where some charge remains upon the residual drop. This dropcan be equated with the primary drop described throughout the presentinvention (see FIGS. 12 and 14). If this drop is permitted to passthrough a ring shaped electrode of a type similar to the extractor ringsituated at some distance between the capillary nozzle and thecollector, and connected to ground or to a negative potential, then someelectrostatic force is exerted upon the drop as it passes through thevicinity of the ring plane. If this force is sufficient to causedistortion of the drop, then the middle of the drop may be constrictedsufficiently that surface tension forces will act along a path of leastresistance at the neck by forming two (or possibly more) secondarydrops.

Thus, this process can be repeated as the droplets pass through otherstrategically placed rings. However, as the mass of the initial drop issubdivided, so the charge on each droplet is reduced also (i.e., thecharge to mass ratio is reduced) and some charge is lost to theimmediate environment due to leakage. The drops may regain some positivecharge by induction as they pass down the electrostatic force field, butthe charge effect will reduce with distance from the capillary nozzle,and the effect will be further reduced as the rings through which thedrops have already passed act to nullify the electrostatic field. Anydroplets produced in this manner would be third stage, or tertiarydrops.

Thus, the possibility exists for successive atomization of a drop ofliquid metal by a DC. electric field provided that certain criteria aremet.

One criterion is reducing the flow rate of the liquid metal bycontrolling the high voltage that is applied to the reservoir. Thispermits the drop a slower passage through the ring system and somaximizes the atomization effect.

Another criterion is positioning the ring electrodes with successivelynegative and positive potentials in sandwich fashion such that the dropsare alternately atomized, and then recharged by induction. For example,a ring that is funnel-shaped may be used to investigate whether exposingthe liquid metal drop to an intense field for a longer period canincrease the charge imparted by induction, and also whether such afunnel-shaped electrode, when connected to a polarity opposite to thatinduced upon the drop will permit greater atomization to occur. However,in this situation there has to be some trade-off between the drop'sdownward velocity and the potential applied to the funnel electrodes.Too high a charging voltage on the electrode will retard the drop'sability to leave the nozzle with minimum mass, while too little voltagewill produce a shorter flight time in the shearing field. To some extentthese effects can be minimized by varying the positions of the ringelectrodes along the drop's flight path, and it is these effects that weare currently studying.

Another criterion is maintaining a heated flight path that is longerthan previously employed in order to facilitate the layering of rings.This flight path should be maintained at a temperature which ensuresthat all constituents of the eutectic alloy remain liquid. Though theeutectic isotherm for pure Wood's Metal is 70° C., that for a recycledalloy, where the relative constituency may have changed, may beconsiderably higher.

At some stage successive atomization will no longer occur. Intuitively,there may be a limiting size of drop which cannot be further subdividedby electrostatic shearing. As the drop becomes smaller, there is lessopportunity for the electrostatic forces to form a distorted shape whichcan then usurp the surface tension forces to help form smaller dropletsbecause as the drop's mass becomes smaller there will be a tendency forthe drop to ‘float’ on the field rather than be severed by it.

Tables 2 and 3 in Example 3 indicate that a finite number of groups(ostensibly 3 or 4) containing drops of similar sizes resulted from eachexperimental sample. While examining FIG. 14, we can understand why thissituation occurs. As the partially charged drop falls through the ringelectrodes, electrostatic shearing forces act upon the drop. However,imperfections in construction mean that the ring is not exactlyconcentric to the drop's flight path, therefore the electrostatic fieldintensity acting upon the drop is not totally uniform. The drop is notdivided exactly in a manner of binary division. Instead, two or moredroplets of unequal mass are produced from each drop that is emittedfrom the nozzle. This produces samples containing the grouping that wehave witnessed so far. Our aim therefore is to provide an opportunityfor sufficient successive atomizations to occur until the requisiteparticle size distribution is achieved.

In preferred embodiments, the atomization methods and apparatus furthercomprise non-equilibrium plasmas for removing heat from the moltenparticles after they are electrostatically atomized but before they arecollected either as a solid workpiece or as a powder. Alternatively,non-equilibrium plasmas can be used to remove heat from the moltenparticles after they are applied to a substrate. FIGS. 26-34 showvarious preferred embodiments. In particular, FIGS. 26-29 provideexamples of methods of making a molten metal stream or droplets in avacuum for atomizing, while FIGS. 30-34 provide examples of methods usedto collect the atomized liquid metal in a vacuum. The atomizationmethods, electrostatic methods, and non-equilibrium plasmas described inFIGS. 26-34 are preferably those of the present invention, as describedherein.

FIG. 26 shows twin electrode melting as the source for the molten metalfor electrostatic atomizing. The vacuum chamber 501 surrounds theelectrodes 503 and the atomizing source 505. Molten metal 504, either asdroplets or a stream, falls from the electrodes 503 to the electrostaticatomizer 505. The atomized material 506 flows out of the atomizer 505and into a collecting means (not shown), examples of which are describedin FIGS. 30-34.

FIG. 27 shows electron beam melting as the source for the molten metalfor electrostatic atomizing in vacuum. The vacuum chamber 501 surroundsthe electron beam source 502, the electrode 503, the atomizing source505 and the collector (not shown). Molten metal 504, either as a streamor droplets, falls from the electrode 503 to the electrostatic atomizer505. The atomized material 506 flows from the electrostatic atomizer 505into a collection means (not shown), examples of which are described inFIGS. 30-34.

FIG. 28 shows electron beam cold hearth melting as the source for moltenmetal for electrostatic atomizing in vacuum. The vacuum chamber 501surrounds the electron beam source 502, the electrode 503, thewater-cooled copper cold hearth 507, the atomizing source 505, and thecollection device (not shown). Molten metal 504, either as a stream ordroplets, falls from the water-cooled copper cold hearth 507 to theelectrostatic atomizer 505. The atomized material 506 flows from theelectrostatic atomizer 505 into a collection means (not shown), examplesof which are described in FIGS. 30-34.

FIG. 29 shows ESR/CIG melting as the source for the molten metal forelectrostatic atomizing in vacuum. Alternatively, a VAR/CIG melt sourcemay be used in place of the ESR/CIG melt source. The vacuum chamber 501surrounds the melt source, the electrostatic atomizer 505 and thecollection device (not shown). The ESR/CIG melt source includes anelectrode 503 and a water-cooled copper crucible 507. A molten slag 508acts to melt the electrode 503 to form a molten metal pool 509. Themolten metal 204, either as a stream or droplets, flows through the CIGnozzle 510, and falls into the electrostatic atomizer 505. The atomizedmaterial 506 flows from the electrostatic atomizer 505 into a collectionmeans (not shown), examples of which are described in FIGS. 30-34.

Throughout the description of FIGS. 26-34, the molten metal 504 ispreferably atomized using the methods described herein.

FIG. 30 shows the atomized powder being collected in the bottom of theatomizing chamber. The vacuum chamber 501 contains a melting andatomizing means described in FIGS. 26-29. The stream or droplets ofmolten metal 504 from the melt sources described in FIGS. 26-29 passesthrough the atomizing zone 511. The atomized material 506 is collectedat the bottom of the chamber 512.

FIG. 31 shows electrostatically atomized powder being collected as asolid preform after the powder is cooled via a non-equilibrium plasma.The vacuum chamber 501 contains a melting and atomizing means describedin FIGS. 26-29. The stream or droplets of molten metal 504 from the meltsources described in FIGS. 26-29 passes through the atomizing zone 511.The atomized powder 514 passes through a non-equilibrium plasma 515 andis collected as a solid preform 516. The non-equilibrium plasma 515 isgenerated by producing a potential difference between two electrodes 503from a power source 517. The heat from the atomized powder 514 isconducted through the non-equilibrium plasma 515 and the electrode 503into a dielectric heat transfer medium to a heat exchanger 518.

FIG. 32 shows electrostatically atomized powder being collected in acan, where the can is transferred into a smaller chamber withoutbreaking the vacuum. In the smaller chamber, the lid may welded to thecan prior to hot working to a final product. The vacuum chamber 501contains a melting and atomizing means described in FIGS. 26-29. Thestream or droplets of molten metal 504 from the melt sources describedin FIGS. 26-29 passes through the atomizing zone 511. The atomizedpowder 514 is directed into a can 519 via the process described in FIG.34. When the can 519 is sufficiently full of atomized powder 514, it istransferred in the chamber 520 and the chamber 520 is sealed by a vacuumlock 521. A lid can then be applied to the filled atomized powder canand the can released to the atmosphere via a second lock 521B forthermomechanical processing.

FIG. 33 shows the production of a solid ingot in a mold from a powderproduced by electrostatic atomization. The vacuum chamber 501 contains amelting and atomizing means described in FIGS. 26-29. The stream ordroplets of molten metal 504 from the melt sources described in FIGS.26-29 passes through the atomizing zone 511. The atomized powder 514 iscollected in a mold 522 and the solid ingot 524 withdrawn from the mold522. Power supplies 517 provide the potential difference to form anon-equilibrium plasma 515 emanating from the electrodes 503. Heat isconducted from the surface of the solidifying ingots 524 to theelectrodes 503 which are cooled with a dielectric liquid. The liquid ispassed through heat exchangers 518 and returned to the electrodes 503.

FIG. 34 shows three stages of electrostatic atomizing using plasma andone stage of electrostatic steering of the atomized powder. The vacuumchamber 501 contains a melting and atomizing means described in FIGS.26-29. The stream or droplets of molten metal 504 from the melt sourcesdescribed in FIGS. 26-29 passes through the atomizing zone 511. Thenon-equilibrium plasma 515 for imparting the atomizing conditions isprovided by the potential difference between the electrodes 503. Thepotential difference is supplied by a high-voltage power supply 517. Theatomized material from the first stage 525 passes to the secondatomizing stage, and atomized materials of smaller size from the secondstage 526 pass to the third stage. Atomized materials from the thirdstage 527 pass through the steering stage to be steered in a directionwhich depends on the potential between the electrodes 503. Power forthese electrodes is supplied by power supply 517.

Using various features described above, it would be readily apparent toone of ordinary skill in the art that the following exemplaryembodiments can be implemented. Of instance, in one embodiment, thepresent invention describes apparatus comprising dispensing means,collecting means, and means for directing molten particles from thedispensing means to the collecting means comprising an electrostaticfield and/or an electromagnetic field. Optionally, the apparatus mayfurther comprise atomization apparatus and/or non-equilibrium heattransfer apparatus.

In another embodiment, the present invention describes spray formingmethods comprising directing molten particles from dispensing means tocollecting means by producing an electrostatic field and/orelectromagnetic field between the dispensing means and the collectingmeans. Optionally, the apparatus may further comprise atomizationapparatus and/or non-equilibrium heat transfer apparatus.

In another embodiment, the present invention is directed to apparatuscomprising a melt chamber that comprises at least one orifice; a meansfor expelling a molten material through the at least one orifice in themelt chamber; and a means for applying a rapid electrostatic charge tothe molten material. Preferably, the means for forcing the moltenmaterial through the at least one orifice in the melt chamber is amechanical or electromechanical actuator or a pressure means. In apreferred embodiment, the apparatus further comprises a means forcooling the molten particle. Preferably, the means for cooling themolten particle comprises a means for generating a non-equilibriumplasma.

In another embodiment, the present invention describes methods forforming particles comprising producing a first molten particle; andapplying a rapid electrostatic charge to the first molten particle,wherein the rapid electrostatic charge causes the first molten particleto form at least one smaller second particle. Preferably, the firstmolten particle is expelled through at least one orifice in the meltchamber via mechanical means or by a pressure means. In a preferredembodiment, the at least one smaller second molten particle is cooled,preferably by a non-equilibrium plasma.

In another embodiment, the present invention is directed to apparatusfor transferring heat between a heat-transfer device and a workpiececomprising the heat-transfer device, wherein the heat-transfer device iselectrically charged or held at a potential; the workpiece, wherein theworkpiece is mechanically separate from the heat-transfer device; andmeans for transferring heat between the workpiece and the heat-transferdevice comprising a means for generating a non-equilibrium plasma. Theheat-transfer device can be either a heat sink or a heat source.

In yet another embodiment, the present invention is directed to methodsof transferring heat between a heat-transfer device and a workpiececomprising producing a non-equilibrium plasma capable of transferringheat between the heat-transfer device and the workpiece, wherein theheat-transfer device is electrically charged or held at a potential, andwherein the heat-transfer device is mechanically separate from theworkpiece. The heat-transfer device can be either a heat sink or a heatsource.

Accordingly, in various embodiments, non-equilibrium plasmas areadvantageously employed to effect optimal heat transfer, and thenon-equilibrium plasma must act with a heat sink/source that has athermal conductivity capable of removing the desired quantity of heat.While two or more electrodes have been used in the past to produce aplasma in a region of high heat, such as a weld zone, so that the plasmawould serve to conduct heat outward from the weld zone, therebyincreasing the surface area for heat, embodiments of the presentinvention are directed to the discovery that a non-equilibrium plasmamay be used to introduce heat into a workpiece as well as from aworkpiece. It has further been surprisingly discovered that under thecorrect conditions a non-equilibrium plasma can be used to efficientlytransfer heat in a vacuum.

The novel methods of the present invention are particularly useful inpreparing any metal article, such as articles for gas turbine engines,including, for example, airfoils, blades, discs and blisks.

Accordingly, in one aspect, there is provided according to the presentinvention an apparatus comprising: a dispensing means; a collectingmeans; and a means for directing a molten particle from the dispensingmeans to the collecting means comprising at least one of anelectrostatic field or an electromagnetic field. In another aspect isprovided the apparatus described above, wherein the means for directingthe molten particles from the dispensing means to the collecting meanscomprises an electrostatic field or an electromagnetic field. Theapparatus may further comprise at least one magnetic coil, and may alsofurther comprise a means for charging the molten particles. In oneembodiment, the means for charging the molten particles may comprise athermionic emission source or a tribocharging device. The dispensingmeans of the apparatus may be a gas atomizer, and may further comprise ameans for transferring heat from the molten particles. The means fortransferring heat from the molten particles may comprise gas conductionand/or convection and/or a non-equilibrium plasma.

In another aspect, there is provided according to the present inventionan apparatus comprising: a dispensing means; a collecting means; and ameans for directing a molten particle from the dispensing means to thecollecting means comprising at least one of an electrostatic field or anelectromagnetic field, and further comprising a means for transferringheat from the collecting means. The means for transferring heat from thecollecting means may comprise a means for generating a non-equilibriumplasma. In a particular aspect, the means for transferring heat from themolten particles comprises a first heat sink, wherein the first heatsink is electrically charged or held at a potential; and a means fortransferring heat from the molten particles to the first heat sinkcomprising a means for generating a non-equilibrium plasma. Thenon-equilibrium plasma may be a glow discharge or a cold coronadischarge.

In another aspect, there is provided according to the present inventionan apparatus comprising: a dispensing means; a collecting means; and ameans for directing a molten particle from the dispensing means to thecollecting means comprising at least one of an electrostatic field or anelectromagnetic field, and further comprising a means for expelling themolten particle through at least one orifice in the dispensing means;and a means for applying a rapid electrostatic charge to the moltenmaterial. The means for expelling the molten particle through the atleast one orifice may comprise a mechanical or electromechanicalactuator. In one aspect, the means for expelling the molten particlethrough the at least one orifice may be a pressure means that produces apressure in the dispensing means that is greater than the pressure onthe outside of the dispensing means. The pressure means may causeinterrupted flow of the molten particle from the dispensing means. Therapid electrostatic charge may be an arc discharge or an electron beam.

In another aspect, the present invention provides for a spray formingmethod comprising directing molten particles from a dispensing means toa collecting means by producing at least one of an electrostatic fieldor an electromagnetic field between the dispensing means and thecollecting means. The electromagnetic field may be produced by, forexample, means comprising at least one magnetic coil. The methodaccording to this aspect of the invention may further comprise chargingthe molten particles. Charging the molten particles may be accomplished,for example, using a thermionic emission source or a tribochargingdevice. In one aspect, the dispensing means may be a gas atomizer.According to this aspect of the invention, the method may furthercomprise transferring heat from the molten particle. Transferring heatfrom the molten particles may be accomplished, for example, by gasconduction and/or convection and/or non-equilibrium plasma. In anotheraspect, the method of the invention further comprises producing a secondelectromagnetic field. According to the invention, the method mayfurther comprise transferring heat from the collecting means, which maybe by a non-equilibrium plasma.

In another aspect, the present invention provides for a spray formingmethod comprising directing molten particles from a dispensing means toa collecting means by producing at least one of an electrostatic fieldor an electromagnetic field between the dispensing means and thecollecting means, further comprising applying a rapid electrostaticcharge to the molten particle, wherein the rapid electrostatic chargecauses the molten particle to form at least one smaller molten particle.In a particular aspect, the rapid electrostatic charge may be an arcdischarge or an electron beam. In another aspect, the method of theinvention may further comprise transferring heat from the moltenparticle comprising producing a non-equilibrium plasma that transfersheat from the molten particle to a first heat sink, wherein the firstheat sink is electrically charged or held at a potential. Thenon-equilibrium plasma may be a glow discharge or a cold coronadischarge.

In another aspect, the invention is directed to an apparatus comprisinga melt chamber comprising at least one orifice; a means for forcing amolten material through the at least one orifice in the melt chamber;and a means for applying a rapid electrostatic charge to the moltenmaterial. The rapid electrostatic charge may be an arc discharge or enelectron beam. The apparatus of the invention may further comprise ameans for cooling the molten material. In a particular aspect, the meansfor cooling the molten material may comprise a first heat sink, whereinthe first heat sink is electrically charged or held at a potential; anda means for transferring heat from the molten material to the first heatsink comprising a means for generating a non-equilibrium plasma. Thenon-equilibrium plasma may be a glow discharge or a cold coronadischarge.

In another aspect, there is provided a method for atomizing a particlecomprising producing a first molten particle; applying a rapidelectrostatic charge to the first molten particle, wherein the rapidelectrostatic charge causes the first molten particle to form at leastone smaller second molten particle. According to the method of theinvention, the first molten particle may be produced by melting amaterial in a melt chamber, and expelling the first molten particlethrough at least one orifice in the melt chamber. The rapidelectrostatic charge may be an arc discharge or en electron beam. Themethod of the invention may further comprise cooling the second moltenparticle by producing a non-equilibrium plasma that transfers heat fromthe second molten particle to a first heat sink, wherein the first heatsink is electrically charged or held at a potential. The non-equilibriumplasma may be a glow discharge or a cold corona discharge.

In another aspect, the invention provides for an apparatus fortransferring heat between a first heat-transfer device and a workpiececomprising a first heat-transfer device, wherein the first heat-transferdevice is electrically charged or held at a potential, and wherein thefirst heat-transfer device is a heat sink or a heat source; a workpiece,wherein the workpiece is mechanically separate from the firstheat-transfer device; and means for transferring heat between theworkpiece and the first heat-transfer device comprising a means forgenerating a non-equilibrium plasma. The non-equilibrium plasma may be aglow discharge or a cold corona discharge. The apparatus of theinvention may further comprise an external means for generating ormaintaining the non-equilibrium plasma. The external means forgenerating or maintaining the non-equilibrium plasma may be a thermionicemission, an RF electromagnetic radiation, an electromagnetic radiation,a magnetic field or an electron beam. The first heat-transfer device ofthe apparatus of the invention may comprise a plurality of heat-transferdevices. In a particular aspect, the apparatus of the invention mayfurther comprise a second heat-transfer device that may be mechanicallyand electrically separate from the first heat-transfer device, whereinthe second heat-transfer device is a heat sink or a heat source, andwherein the potential between the first heat-transfer device and thesecond heat-transfer device produces a non-equilibrium plasma.

In another aspect is provided a method for transferring heat between afirst heat-transfer device and a workpiece comprising producing anon-equilibrium plasma that transfers heat between the firstheat-transfer device and the workpiece, wherein the first heat-transferdevice is electrically charged or held at a potential, wherein the firstheat-transfer device is mechanically separate from the workpiece, andwherein the first heat-transfer device is a heat sink or a heat source.The non-equilibrium plasma may be a glow discharge or a cold coronadischarge. The method may further comprise generating or maintaining thenon-equilibrium plasma via an external means. In an aspect, the externalmeans for generating or maintaining the non-equilibrium plasma comprisesa thermionic emission, an RF electromagnetic radiation, anelectromagnetic radiation, a magnetic field or an electron beam.

In another aspect, the invention provides for a preform produced by themethods of the invention. The preform of the invention may be a near netpreform. There is also provided an article of manufacture produced bythe method of the invention.

In another embodiment, present invention includes an apparatuscomprising, a dispenser, a collector, a means for directing a moltenparticle from the dispenser to the collector comprising at least one ofan electrostatic field or an electromagnetic field, and a heattransferring device from the collector comprising a non-equilibriumplasma generator.

In yet another embodiment, the present invention includes an apparatuscomprising, a dispenser, a collector, a means for directing a moltenparticle from the dispenser to the collector comprising at least one ofan electrostatic field or an electromagnetic field, and heattransferring device from the molten particle comprising anon-equilibrium plasma generator.

EXAMPLES

The following examples are for purposes of illustration only, are notindented to bind the scope of the present invention to any particulartheories or embodiments described therein, and are not intended to limitthe scope of the invention or the appended claims.

Example 1

Clean metal spraying experiments revealed that Wood's metal could easilybe charged positively or negatively. It was observed that as the meltingpoint of the metal increased, the ability to positively charge the metaldid not change or improved slightly, while the ability to negativelycharge the metal decreased. It was determined that the metals werepositively charged to about 78% of the Rayleigh limit.

This example demonstrates that as the temperature of the metalincreases, the electron emission rate increases. Thus, the ability topositively charge the metal is unaltered or improved, while the abilityto negatively charge the metal is reduced.

Example 2

In this example, the feasibility of deflecting charged metal particlesin a controlled and repeatable manner using an electrostatic field wasinvestigated. To this end, molten metal particles comprising tin werepositively charged and then passed within 2 cm of an electrostaticallycharged plate. The particle sizes were about 0.050 inches to about 0.250inches. The polarity and magnitude of the charge on the plate was variedin different trials.

The results indicated that the positively charged plate repelled thepositively charged particles and the negatively charged plate attractedthe metal particles. The deflection characteristics versus the appliedvoltage are shown in FIG. 5 for the molten particles comprising tin.

Example 3

In this example, a video tape data analysis was developed to analyze theresults obtained in Example 2 to provide a statistically viablecomparison between the video of the spray forming process with andwithout an electrostatic field applied.

An 8 mm video tape was digitized and replayed frame by frame on a highcontrast NTSC video monitor. Each frame was judged as demonstrating goodor poor collection efficiency based on three criteria. (1) Attenuation:If less than 80% of the particles directly targeted the preform then theframe was judged as demonstrating poor collection efficiency. (2)Bounce: If the particles appeared to bounce off the preform then theframe was judged as demonstrating poor collection efficiency. (3) Glow:If the particles produced a glow over the preform, it was indicative ofa combination of poor attenuation and bounce off, and was judged asdemonstrating poor collection efficiency.

After the frames were categorized as demonstrating good or poorcollection efficiency, groups of frames were selected for analysis. Theframes for analysis were chosen based on a review of the strip chart ofthe voltage and current. An electrostatic event zone was defined as aperiod of time wherein high voltage was being applied and current wasbeing drawn. Thus, any momentary changes in the collection efficiencycould be eliminated from consideration in the experiment by comparingthe number of frames demonstrating good collection efficiency versus thetotal number of frames in the electrostatic event zone as compared tothe neutral zones. It was estimated that the synchronization between thevideo tape of the clean metal spray run and the strip chart wereaccurate within ±0.3 seconds. Thus, 0.3 seconds were added to thebeginning and end of each electrostatic event to ensure that the eventwas fully captured. Three electrostatic event zones were found andselected for analysis. Three neutral zone periods, where no current wasdrawn, were selected for comparison controls.

The results of the analysis of the experimental data are shown in Table1 below. The results in Table 1 indicate that in neutral Zones 1, 2 and5, an average of 13.7% of the frames demonstrated good collectionefficiency. In Zone 6, where an average of 198 watts of electrostaticpower was applied, 31.6% of the frames demonstrated good collectionefficiency. In Zone 4, where an average of 245 watts of electrostaticpower was applied, 39.3% of the frames demonstrated good collectionefficiency. In Zone 3, where an average of 2000 watts of electrostaticpower was applied, 59.3% of the frames demonstrated good collectionefficiency.

This example shows that a significant improvement in the number offrames demonstrating good collection efficiency corresponded with theelectrostatic events, that the improvement in the number of framesdemonstrating good collection is proportional to the power drawn, andthat the significant improvement observed is not merely additive.

Example 4

This experiment demonstrated successful atomization of liquids (coloredwater, molten Wood's metal) using an apparatus according to theinvention comprising a high voltage DC source. The apparatus permittedlimited atomization of liquid metal when the flow was in an upwarddirection against gravity, or in a downward direction with gravity. Anumber of controlling parameters such as nozzle size, ambientconditions, spacing between electrodes, dielectric medium betweenelectrodes, and shape and size of extractor electrode were varied suchthat atomization of liquid metal was substantially effected using only aDC source. Atomization involving DC alone according to the invention wasexemplified by demonstrating (i) liquid metal flow against the directionof gravity; and (ii) liquid metal flow in the direction of gravity.

The apparatus used for liquid metal flow against the direction ofgravity is shown in FIGS. 15 and 15A. FIG. 15A shows the liquid metalreservoir 801, a connecting tube 802, a nozzle 803 and an extractor ring804. A copper reservoir was connected to the nozzle assembly by means ofa copper tube. Solid pieces of eutectic alloy (Wood's metal) were placedin the reservoir and heated to approximately 100° C. by radiation fromtwo 150 watt halogen bulbs. A pressure difference between the widerreservoir and the nozzle caused the liquid to flow to the tip of thenozzle, and the flow rate from the nozzle was regulated by the amount ofalloy placed in the reservoir. The reservoir and nozzle assembly wasconnected directly to the high voltage terminal. A copper spheroid whichwas placed within {fraction (1/16)}th of an inch from the nozzle tipsurrounded the replaceable nozzle, and a copper tube thermally connectedthis spheroid to the reservoir assembly. In this way, not only heat wasimparted to the eutectic alloy by conduction along the majority of itspath, but also a charge buildup area was extended. The whole arrangementwas painted black to behave as a black body absorber. A brass extractorring was suspended at varying distances directly above the nozzleopening and was connected to a grounded electrical terminal by means ofan adjustable mounting assembly.

For liquid metal flow in the direction of gravity, a similar copperreservoir was connected to a nozzle assembly by means of a shortdownward tube of thinner internal diameter than the reservoir, and thisarrangement was attached directly to the positive high voltage terminal.An extractor ring was placed at varying distances below, or to the sideof, the nozzle tip, and a collector cup, placed directly below thenozzle and partially filled with water, collected atomized samples.

A sensitive electronic balance was used to weigh drops and droplets fromatomization experiments. Control experiments were performed when dropswere produced by gravitational force and free hydrostatic pressure (fordownward liquid) alone. Repeat experiments were then conducted using asimilar head of molten metal, and a high voltage (max 36 keV, max 0.2mA) was applied to the fluid. Equal numbers of drops and droplets(nominally 100) were weighed, and their weights compared. FIGS. 16 and17 show the drops and droplets collected from one such series ofexperiments. For each figure, the larger drops (upper portion of thefigure) are those collected during the control experiments, and thesmaller droplets (lower portion of the figure) are those collectedduring experiments using electrostatic field.

It was observed that atomization of a liquid metal utilizing an electricfield between two electrodes alone was hampered by two over-ridingfactors: (a) in air, at atmospheric pressure, arcing occurred at avoltage lower than that required to atomize the liquid metal byelectrostatic means; and (b) at pressures slightly less thanatmospheric, there was an inability to create potential differencesbetween the electrodes sufficiently high enough to enable atomization tooccur. The reason for this is thought to be that the formation of plasmapermitted an easy path for current flow.

In order to address these factors and the resultant uncontrolled voltagebreakdown, a preferred embodiment of the invention employed a piece ofCPVC pipe or other dielectric material 825, preferably {fraction(1/8)}th of an inch thick, placed in such a way as to surround theextractor ring and its supporting arm 804 (see FIGS. 18A and 18-25). Byincorporating this CPVC 825 into the assembly, substantially higherpotential differences between nozzle 803 and extractor 804 could beachieved before arcing occurred. This meant that more electrical energywas available for atomization, though the distance between theelectrodes could only be increased to a point where leaking would occurto neighboring components.

For liquid metal flow against the direction of gravity, the nozzleaperture that permitted the best atomization was on the order of 0.3 to0.4 mm, which is accordingly a preferred nozzle aperture size for thispurpose. For apertures smaller than this, difficulty was encountered insecuring a liquid metal flow. Apertures larger than this permitted toogreat a surface area of liquid metal at the nozzle tip, which could notbe satisfactorily atomized using the particular experimental apparatus.It will be appreciated that larger apertures may be achieved by varyingthe design of the apparatus, with the exercise of no more than routineskill once the present teachings are understood. Liquid metalatomization was captured on video camera, but could only continue for aslong as the head of pressure in the reservoir permitted flow of fluid tothe nozzle tip. A prelude to atomization could be witnessed as highvoltage was increased by either an increase in fluid flow rate, or as avertex (Taylor Cone) forming on the surface of the liquid metal at thetip of the nozzle. Although not intending to be bound by any particulartheory, it may be that if too long a time elapses between achieving acomplete flow of fluid from the reservoir to the nozzle tip andincreasing the high voltage from zero to a level sufficient to permitatomization, then satisfactory atomization would not occur. Instead,cooling and oxidation of the free liquid surface would be apparent andthe liquid metal would flow downward. The potential difference betweenthe nozzle and the grounded steel base plate was sufficient to increasethe liquid flow. Thus, a “pipe” of solidified metal would slowly formdown the side of the nozzle assembly until the pressure difference atthe reservoir decreased to zero.

Following this approach, the inventors successfully achieved atomizationby means of a combination of mechanical and electrical forces. In anexemplary experiment, a plunger was placed over the open end of thereservoir to increase the pressure difference at the nozzle tip. It wasfound that production of droplets by electrostatic atomization requiredless potential difference between the electrodes, and droplet sizes weresmaller.

In another experiment, the eutectic alloy was removed from the reservoirassembly and was replaced by water. The water was then drained from theassembly, leaving the inner walls wet. The eutectic alloy was thenreturned to the reservoir and reheated. When liquid metal flow from thenozzle occurred, drops were ejected by the force of steam that wastrapped within the nozzle assembly. When a high voltage was applied, thedrops became atomized into smaller droplets, some of which adhered tothe underside of the CPVC.

FIGS. 18, 19 and 20 show consecutive frames of the atomization of liquidmetal against gravity without any applied mechanical force other thanthat due to the head of liquid in the reservoir. It was found thatliquid metal flow in this direction was controlled more easily.Conditions could be created such that a continuous flow of molten metalwas produced and nozzle apertures could be much smaller. Nozzles havingpreferred aperture sizes in the range of about 0.1 mm to about 10 mm,more preferably, about 0.15 mm to about 2 mm, yet more preferably, about0.15 mm to about 0.4 mm, and even more preferably, about 0.15 mm toabout 0.3 mm, most preferably about 0.15 mm, are employed. It wasobserved in use that gravitational forces alone could not overcome theadhesive forces of surface tension upon the inner walls of the nozzle,and the cohesive forces within the surface of the metal. In suchinstances, it was found that electrostatic atomization could producedroplets with diameters of substantially smaller magnitude.

The atomized droplets produced without applied external force weresubstantially larger than the aperture of the nozzle. For example, usinga nozzle with an aperture diameter of 0.5 mm and an outer diameter of1.2 mm, the observed droplet diameter was 1.4 mm. Generally, smallernozzle sizes permitted the production of smaller drops to be emittedfrom the nozzle.

Drops produced by gravitational forces alone were irregular in size andare not completely spherical. Droplets produced when electrostaticatomization occurs were found to demonstrate a more regular sizedistribution and were more nearly spherical. It is believed that gravityacts upon the mass of liquid within the reservoir and nozzle assembly toproduce a tendency for liquid flow. Application of high voltage produceda force field which combined with gravity to produce a greater tendencyfor liquid flow.

The liquid metal drops emitted from the nozzle were charged byelectrostatic means. Positioning the extractor electrode in a coplanarposition with the tip of the nozzle appeared to produce the strongestfield for electrostatic atomization to occur. The application of DC highvoltage to a liquid metal source was found to produce atomized droplets.The size distribution of droplets emitted from the nozzle was determinedby nozzle size, hydrodynamic and electrostatic forces. If the chargeupon the drops was sufficiently high, there was evidence to suggest thatthey may be subject to binary division by purely electrostatic means.For a nozzle with an outer diameter of 0.64 mm, the measured weightratio of drops produced without high voltage to those produced with highvoltage was approximately 2:1. For a nozzle with an outer diameter of0.40 mm, the measured weight ratio increased to approximately 4:1.

As the aperture of the nozzle was decreased, the size of the dropletsproduced decreased rapidly, indicating that the fluid emitted wassevered before surface tension forces caused it to adhere to the nozzletip. These observations suggest that electrostatic forces begin topredominate over mechanical forces. The plume of spray evident in FIG.24 indicates that liquid metal ionization may have occurred as a resultof a Taylor Cone being produced on the surface of the fluid that isbeing emitted from the nozzle. As electrostatic forces play a moredominant role in the atomization process, a high rise-rate AC signal maybe advantageously incorporated into the system to create an extremelyefficient method of producing high quality pure metallic grains ofsimilar size.

Example 5

Practical difficulties arise when cooling molten or near-molten metalsunder low pressures. Heat transfer from a metal surface into a gasmedium happens via convection, radiation, and conduction. For manylow-pressure and near-vacuum applications, heat transfer by convectionis negligible, while radiative heat transfer alone may be insufficient.According to the present invention, a solution is provided to thisproblem by increasing the effective thermal conductivity of the gasmedium by introducing non-equilibrium ionization in the gas medium. Thismay be carried out at low pressure and under near-vacuum conditions. Thenon-equilibrium plasma thermal conductivity gain achieved by the presentinvention was demonstrated by a series of experiments performed in avacuum chamber which provided comprehensive temperature decaycharacteristics data under a variety of conditions.

With reference to FIGS. 35-36, a stainless steel disk, Block A 601, washeated using a 1 kW Calrod element 614. A large stainless steelcylinder, Block B 602, was located at a distance of 4 inches from BlockA 601 and functioned primarily as an electrode, and secondarily as aheat sink. FIG. 35 shows a schematic diagram of the experimental setup,and FIG. 36 shows the dimensions of the blocks. FIG. 35 shows the vacuumchamber 603, the high voltage connection to Block A 604, the HDPEinsulators 606, Block A with Calrod element and ceramic cover 607, thesteel bases for Blocks A and B 608, the thermocouple terminals 609, theceramic tube supports 610, the terminals for the thermocouple and Calrodelement 611, the thermocouples 612, the ground connection to Block B605, and the viewing port 613. FIG. 36 shows the relationship betweenBlock A 601 and Block B 602, the holes for the thermocouples 616, theCalrod element 614, the ceramic cover 615, the ceramic tube supports610, and the steel bases 608.

This example presents the results of heat transfer gain in anon-equilibrium plasma. In this experiment, an ORAM high voltage powersupply (model number DSR 100-100-JTTF, input: 240V AC single phase, 60Hz, output: 0 to −100 kV rectified) provided the non-equilibrium plasmabetween the blocks. Throughout all experiments, the Calrod element 614remained attached to the smaller block A 601, while the larger block B602 continued to absorb a portion of the heat conducted from block A.Generally, block A 601 was connected to the high voltage negativeterminal 604 and block B 602 was grounded 605, although a fewexperiments were performed to examine the effect of polarity reversal onheat decay.

In order to obtain comprehensive information for study, severalvariables were introduced into the experimental plan. These includeddifferent gases (air, Ar, He), different vacuum pressures (atmosphere,10⁻¹ and 10⁻² Torr), different plasma currents (up to 25 mA and/orvoltages up to 50 kV), and different spacings between the block faces(1½ inches and 4 inches). For comparison of data, attempts were made toensure that two parameters were held reasonably constant (1) the initialtemperature of block A at the start of the decay process, and (2) theduration of the heat decay measurement process.

A non-intrusive instrument for continuously measuring the temperature ofthe high-voltage block from outside the cathode was not available. Aninfrared optical pyrometer obtained for this purpose provedunsatisfactory, hence the necessity of using thermocouples. Even thoughthe voltage control was set low, the high voltage source couldunexpectedly generate a short pulse up to 60 kV when first switched ondue to the type of circuit used within the control unit, and to theinherent cyclical nature of the high voltage generator.

Thermocouples were successfully employed to measure the temperatures ofthe two blocks. However, the temperature of the high voltage block(cathode) could not be monitored continuously. Instead, the high voltagewas switched off at five minute intervals during which temperatures wererecorded from digital multimeters with thermocouple module attachments.The meter arrangement was then disconnected from access terminalsprotruding from the side of the vacuum chamber, and the high voltage wasagain switched on. This measuring procedure meant that the high voltagewas disconnected once for a period of approximately 35 to 40 secondsduring each five minute interval. The Calrod heating element remaineddisconnected during all heat decay measurements. Therefore, it presentedno obstacle to high voltage plasma production.

The complete experimental procedure in air contained the followingstages. The vacuum chamber was pumped down to approximately 10⁻⁴ Torr.The Calrod heater for block A was switched on while the pump cyclecontinued. The heater was switched off close to the required initialtemperature for start of decay measurements. Pumping down of the vacuumchamber was continued until the desired pressure was reached. Theforegoing steps were repeated until the desired pressure and initialtemperature were reached.

Thereafter, the heater was switched off and the initial t=0 details ofblock A temperature, block B temperature, vacuum chamber's top skintemperature, and vacuum chamber pressure were immediately recorded. Thehigh voltage block (cathode) thermocouple meter arrangement wasdisconnected. The exposed terminals were covered with an insulator cap.The high voltage controller was switched on and smoothly brought it tothe required voltage or current. Each minute thereafter, the groundedblock temperature, vacuum chamber top skin temperature, vacuum pressure,applied voltage (in kV) and current (mA) displayed on the high voltagecontrol panel were monitored. At five minute intervals, switched off thehigh voltage supply was switched off, the insulating cap was removed,all terminals exposed from the cap area were briefly grounded, and thethermocouple meter arrangement was connected as described above. Thiswas continued for an elapsed time of 131 minutes.

For experiments in insert gas, one skilled in the art would recognizethat the pumping and heating steps would have to be modified slightly toensure the chamber was filled with gas, and that pressures higher thanrequired were not attained.

In order to test all instruments, plasma tests were carried out in airat atmospheric pressure. This served several purposes: (i) the vacuumchamber door could be left open for observation of plasma or arcing,(ii) the high voltage equipment could be used without the backgroundnoise of vacuum pumps, etc., for detection of voltage breakdown, and(iii) the lighting could be dimmed for better observation of theexperimental area.

The non-equilibrium plasma was first witnessed when the gap between theblock faces was 1½ inches and the voltage was in the 41 kV to 47 kVrange. The non-equilibrium plasma was poorly visible, being blue-greenin color, and was localized and faint. Attempts to increase the currentto a steady flow above 1 mA to achieve more brightness resulted inarcing problems and voltage breakdown. 51 kV was found to be the maximumvoltage that could be applied for steady experimental results, butcurrent flow was less than 0.5 mA, i.e., lower than the multimeter'sdetection threshold. The non-equilibrium plasma was being produced andwas accompanied by a sizzling “boiling oil” sound which began atapproximately 27 kV. FIG. 37 demonstrates the results when 51 kV wasapplied in air at atmospheric pressure.

Attempts to place two point sources (spike) in block B (grounded block)in an effort to improve the visible non-equilibrium plasma volume wereineffective and were not continued. Non-equilibrium plasma was createdat the point tips from 34 kV, being faint blue-green color. The twospikes were removed after these initial tests and not used again.

The first tests in partial vacuum were conducted with all otherconditions being the same. A CCD camera was fixed to the window of thevacuum chamber so that the non-equilibrium plasma could be monitoredthroughout the experiment, and video recorded when desired. The chamberwas pumped down and kept at a steady pressure of 10⁻¹ Torr. At thispressure, extensive non-equilibrium plasma was easily created withvoltages less than 1 kV. A steady purple color was visible and aninitial experiment was conducted with approximately 20 mA current. FIG.38 demonstrates the results achieved during this trial. In order toinvestigate how temperature decay changes, the polarity of the blocksare reversed, the wiring of the block was changed. Block A was groundedand the larger block B was connected to the negative high voltageterminal. All other conditions remained the same, except that thethermocouples were interchanged for safety reasons. The experimental runwas then repeated. The non-equilibrium plasma that was formed wasextensive, purple, and was captured on videotape. FIG. 39 compares thetemperature decay data when block A acted as cathode, against the resultwhen polarity was reversed and block B acted as cathode.

For the remainder of the experimental study, the heated block A was keptat negative potential while the passive block B remained grounded. Thegap between the block faces was next increased to 4 inches, and theresulting effect was examined. This proved to be a more convenient gapto work with because the larger spacing meant that greater plasmacurrents, hence greater electron densities, could be achieved withoutarching, and the plasma region was more visible through the viewingwindow. This became more important at lower pressures, when theoptically bright part of the plasma occupied only a portion of the spacebetween the blocks.

The majority of the remaining tests were therefore carried out with the4 inch gap. By reasoning similar to the above, a larger gap (e.g., 8inches) may have provided even better results, but internal constraintswithin the vacuum chamber prevented this. Given the limitations of theequipment and geometric constraints, the 4 inch gap proved mostpractical. FIGS. 40-42 show the temperature decay curves innon-equilibrium plasma air, argon, and helium using the 4 inch gap,placed along with the coated decay curves.

Since the non-equilibrium plasma heat decay in air at 10⁻¹ Torrdemonstrated results comparable with the best of the other gas mediatested, air was chosen as the medium to examine the effect of varyingcurrent on the heat decay process. Tests were conducted at 10, 15, 20,and 25 mA non-equilibrium plasma currents. Unfortunately, limitations ofthe high voltage power supply controller resulted in frequent arcing andvoltage breakdowns, affecting data acquisition process. As a result,some of the experimental trials were not run to completion. FIG. 43compares the heat decay curves at 10⁻¹ Torr for various non-equilibriumplasma currents.

Feasibility tests in air at pressure less than 10⁻¹ Torr (e.g., 10⁻²,10⁻³ Torr) suggest that higher voltages are required to produce similarnon-equilibrium plasma currents. The low pressure tests alsodemonstrated that the non-equilibrium plasma structure within the blockgap resembled the classical discharge model, with glow column andFaraday dark space becoming more apparent as pressure was decreased. Dueto the limitations of the equipment, maintaining a constant current atthe lower pressures proved to be more difficult, and the arcing problembecame more pronounced.

FIG. 44 shows a graph of the data collected for air at 10⁻² Torr.Although arcing was again a persistent problem, and the data collectionrun was cut short to 40 minutes, it does provide further evidence of thetrend of increased heat transfer with increased current, even at theselow pressures.

The experimental results are summarized in the graphs FIGS. 37-44. Thefollowing is a discussion of some of the features of these graphs. Aparameter called the heat transfer gain coefficient (γ) is introduced toexplain the graphs. In considering a time dependent heat transferproblem where a metal block is giving up heat through a thermallyconductive neutral gas medium, the heat decay curve will be determinedby the thermal diffusivity (α), where α=λ÷(Cρ). C is the heat capacityof the medium at constant pressure, λ is the thermal conductivity, and ρis the density of the medium.

In considering a different gas medium, with conductivity λ′, heatcapacity C′ and density ρ′, then α′=λ′÷(C′ρ′). The heat transfer gaincoefficient between these two systems is defined as γ=α′÷α.

In considering the case of a neutral gas versus a non-equilibriumplasma, for the case of a non-equilibrium plasma with a small ionizationfraction χ<0.1, the presence of electrons hardly affects the density ofthe gas, so ρ′≈ρ.

In considering heat capacity, the molar heat capacity of electrons isextremely small with calculated via Fermi Dirac statistics (Sears, AnIntroduction to Thermodynamics, the Kinetic Theory of Gases andStatistical Mechanics, 2nd Ed., Addison-Wesley, pages 335-337 (1959)).So the contribution of the electrons to C p can be ignored as a firstapproximation. Likewise the contribution of the ionic heat capacity canalso be ignored, again owing to the small ionization fraction.Essentially, this reveals C′≈C. The gain coefficient becomes γ=λ′÷λ.

Since all other system parameters are the same between the two systems(e.g., pressure, metal composition, radiative heat flux, etc.), therereally is no other mechanism by which heat decay time can increase ordecrease other than by gain change γ>1 or γ<1 between the two systems.In other words, γ>1 implies an increase in the thermal conductivity ofthe gas medium. The gain coefficient is a convenient way of comparingsystems with exactly same geometries and similar gas chemistry, onlydifference being the introduction of non-equilibrium ionization. If γ>1between two systems then there is a gain in thermal conductivity due tonon-equilibrium ionization. If γ<1 then there is a net decrease inthermal conductivity due to non-equilibrium effects.

FIGS. 37, 38, 40, 41 and 44 all clearly exhibit (>1, such that thenon-equilibrium plasma effect has been clearly demonstrated and verifiedby these data sets. The effect is most obvious in FIGS. 38, 40 and 41.

FIG. 42, which is a graph of the data for helium at 10⁻¹ Torr is veryinteresting. At first one may be tempted to infer that the effect ispractically nonexistent for He, but that would be a hasty judgment. Infact, this data set is direct evidence for the theoretical mechanism ofnonequilibrium plasma conductivity gain. Consider the data presented inFIG. 41 for argon, and FIG. 42 for helium, both of which are undercomparable pressures and applied power. Ignoring the ionic contributionto 8 as negligible, the gains for these data sets can be writtenrespectively as λ₄₁=(λ_(Ar)+λ_(e))÷λ_(Ar) and γ₄₂=(λ_(He)+λ_(e))÷λ_(He),respectively. The primary factors that affect the gain in thermalconductivity λ are the density of the electrons n_(e), and theelectrons' temperature T_(e). For midrange and large ionizationfractions χ, λ_(e) starts to dominate over the thermal conductivity ofheavy ions and heavy neutrals. In view thereof, a gain in the overallgas thermal conductivity is expected when electrons are supplied at anelevated temperature via a non-equilibrium plasma or corona discharge.

The clear gain visible in FIG. 41 suggests that Xe is not negligible forthe argon case. Now, the first ionization potential for argon is about15.3 electron volts, but for helium it is 35 electron volts (Cobine,Gaseous Conductors, Dover Publications (1958)). Considering thationization kinetics are exponentially dependent on ionization potential,it is entirely reasonable to assume that applied power to argon willproduce more ionization than the same power applied to helium undersimilar conditions. Therefore, λ_(e) for helium should be less thanλ_(e) for argon. Also λ_(He) is an order of magnitude larger than λ_(Ar)to begin with, so the combined effects give λ₄₂<<<<λ₄₁, which is clearlythe case when the data in FIG. 42 is compared to FIG. 41. The practicalimplications of this are as follows: if it is necessary to use helium asthe working medium, the power supply should be modified to compensatefor helium's high ionization potential.

Concerning FIG. 39, this data set exhibits another interestingphenomenon. Considering FIG. 39, it is clear that the gain γ is actuallya function of plasma current. That γ increases with current is notsurprising, since higher currents lead to larger probabilities ofcollisional and cascade ionization, finally leading to higher values ofelectron density n_(e) and ionization fraction χ. What is surprising isthe reversal of γ, i.e., γ<1 when the polarity of blocks A and B arereversed, as can be seen in FIG. 39. In order to gain insight into thispeculiar polarity preference of γ, the full 3×3 thermal conductivitytensor must be considered and it must be determined how the componentsof this tensor depend on the direction of the applied external electricfield vector (Hasse, Thermodynamics of Irreversible Processes, DoverPublications). This polarity anisotropy may have significant value inapplications where one would like to control the anisotropy of thethermal conductivity. TABLE 1 Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6Description neutral neutral alpha beta neutral gamma Voltage none none20-30 kV 35-40 kV 10-2 kV 12-25 kV decaying Current none none 45-100 mA4-7 mA 0 mA 6-16 mA Estimated Wattage none none 2000 W 245 W none 198 WStart Time Stamp 5:00 11:00 22:11 25:02 30:00 42:17 Stop Time Stamp 8:0014:00 24:10 28:01 33:00 45:06 Elapsed Frames 90 90 59 89 90 79 PositiveFrames 7 17 35 35 13 25 % Positive Frames 7.8% 18.9% 59.3% 39.3% 14.4%31.6%

TABLE 2 A set of experimental data for liquid wood's metal atomizationFIRST STATE OF SECOND STAGE OF ATOMIZATION ATOMIZATION PARENT DROPSweight of Weight weight weight of one Of one Extractor No. of of onedrop no. of weight of droplet no. of weight of droplet to source Appliedparent drops (x3) daughter droplets (y3) daughter droplets (z3) Ratio ofRatio of distance Voltage current drops (x2) (gm) droplets (y2) (dm)droplets (z2) (gm) weight weight (mm) (keV) (mA) (x1) (gm) (x2/x1) (y1)(gm) (y2/y1) (z1) (gm) (z2/z1) x3/y3 x3/z3 0 22-25 10 0.255 0.025 860.608 0.0069 4 0 20 10 0.191 0.018 50 0.224 0.0048 4 5 20 30 0.289 0.00960 0.279 0.0046 2 5 25 8 0.163 0.021 60 0.293 0.0048 4 10 25 7 0.1860.019 80 0.366 0.0045 4 15 20 4 0.081 0.021 80 0.384 0.0048 4 0.0050.0012 4 16 15 20 4 0.081 0.021 80 0.384 0.0048 4 0.005 0.0012 4 16 2025 0.035 17 0.304 0.018 109 0.522 0.0047 4 0.008 0.0012 4 15 30 28 0.04815 0.299 0.021 146 0.746 0.0051 4 35 29.5 0.048 11 0.211 0.019 100 0.4890.0048 5 0.003 0.0006 4 32 68 38 3 0.061 0.021 67 0.334 0.0049 7 0.0070.0011 4 20Notes:(1) Parent drop means mechanical drop without electrostatic field.(2) Extractor Type: bare copper wire and 5.2 cm ring diameter.

TABLE 3 A set of experimental data for liquid wood's metal atomizationFIRST STATE OF SECOND STAGE OF PARENT DROPS ATOMIZATION ATOMIZATIONweight of weight of Weight one one Of one Extractor No. of weight dropno. of weight of droplet no. of weight of droplet to source Appliedparent of drops (x3) daughter droplets (y3) daughter droplets (z3) Ratioof Ratio of distance Voltage current drops (x2) (gm) droplets (y2) (dm)droplets (z2) (gm) weight weight (mm) (keV) (mA) (x1) (gm) (x2/x1) (y1)(gm) (y2/y1) (z1) (gm) (z2/z1) x3/y3 x3/z3 15 16 0.014 7 0.136 0.019 1390.66 0.0047 6 0.013 0.0021 4 8 15 16 0.014 20 0.398 0.021 129 0.6470.0051 11 0.023 0.0021 4 9 15 16 0.017 14 0.254 0.025 113 0.572 0.0051 40.007 0.0017 4 10 15 17 0.018 10 0.195 0.021 224 1.111 0.0049 5 0.0060.0012 4 16 15 — — 73 1.454 0.021 58 0.267 0.0049 51 0.107 0.0021 4 1015 17 0.014 4 0.084 0.021 161 0.905 0.0051 4 0.007 0.0017 4 12Notes:(1) Parent drop means mechanical drop without electrostatic field.(2) Extractor Type: bare copper wire and 9 cm ring diameter.

Each patent application and publication referenced herein is herebyincorporated by reference herein in its entirety.

Various modifications of the invention, in addition to those describedherein, will be apparent to one skilled in the art from the foregoingdescription. Such modifications are also intended to fall within thescope of the appended claims.

Example 6

Practical difficulties arise when cooling molten or near-molten metalsunder low pressures. Heat transfer from a metal surface into a gasmedium happens via convection, radiation, and conduction. For many lowpressure and near-vacuum applications, heat transfer by convection isnegligible, while radioactive heat transfer alone may be insufficient.One solution to this problem is to increase the thermal conductivity ofthe gas medium in a non-intrusive manner by introducing nonequilibriumionization in the gas medium. In order to study this nonequilibriumplasma thermal conductivity gain, a series of control experiments wereperformed in a vacuum chamber to establish comprehensive background datafor temperature decay characteristics under a variety of conditions.

Collisions between species is the mechanism by which transport processesoccur in gases. As such the fundamental quantities are the collisioncross sections. Consider ionized Argon with three species: i, a, e,signifying heavy ions, heavy neutrals, and electrons, respectively.Furthermore consider the gas to be in thermal nonequilibrium between theheavy species and the electrons: i.e., the heavy particles arethermalized at a temperature T_(a)=T_(i)=T_(r), while the electrons arethermalized at T_(e)>T. The following are collision cross section modelsfor the case of low pressure and low electron density. Q_(xy) denotesthe collision cross section between species x and species y. Theseexpressions for cross section can be adapted to cover higher n_(g)regimes by working with tabulated data in:Q _(pn)=3.6×10⁻⁴ T _(e)−0.1)×10⁻²⁰  (1)Q _(aa)=1.7×10⁻¹⁶ T ^(−0.25)  (2)Q _(ia)=2.45×10⁻¹⁸ T ^(−0.9)  (3) $\begin{matrix}{Q_{ii} = {2{\pi\left( \frac{e^{2}}{12{\pi ɛ}_{0}{kT}} \right)}^{2}{\ln\left( {\frac{1}{12\pi\quad n_{i}}\left( \frac{12{\pi ɛ}_{0}{kT}}{e^{2}} \right)^{3}} \right)}}} & (4) \\{Q_{ei} = {{\pi\left( \frac{e^{2}}{12{\pi ɛ}_{0}{kT}} \right)}^{2}{\ln\left( {\frac{1}{4\pi\quad n_{e}}\left( \frac{4{\pi ɛ}_{0}{kT}}{e^{2}\left( {1 + {{Ta}/T}} \right)} \right)^{3}} \right)}}} & (5)\end{matrix}$where e is the electron charge, eo is the permittivity of vacuum, k isBoltzmann's constant, and n_(i), n_(e) are the ion and electron numberdensities. All units are MKS. Electron-electron collision are ignored inthe small _(χ) (ionization fraction) approximation.

Collision frequencies can be obtained by combining the relations (1),(6) with the Maxwellian velocities of each species:v _(e) =u _(e)(n _(i) Q _(at) +n _(a) Q _(aa))  (6)v _(i) =u _(i)(n _(e) Q _(et) +n _(a) Q _(ia) +n _(a) Q _(ii))  (7)v _(a) =u _(a)(n _(i) Q _(ia) +n _(e) Q _(ea) +n _(a) Q _(aa))  (8)$\begin{matrix}{{{u_{k} = \sqrt{\frac{8{kt}_{k}}{\pi\quad{nk}}}};{k = i}},a,{e.}} & (9)\end{matrix}$

Thermal conductivity in terms of the collision frequencies is the sum ofthe contributions from all three species:λ=λ_(a)+λ_(i)+λ_(e)  (10)where $\begin{matrix}{\lambda_{o} = {\frac{75\sqrt{2}k^{2}T}{32m_{a}}\frac{n_{a}}{v_{a}}}} & (11) \\{\lambda_{i} = {\frac{75\sqrt{2}k^{2}T}{32m_{i}}\frac{n_{i}}{v_{i}}}} & (12) \\{\lambda_{e} = {\frac{2k_{1}k_{2}k^{- 2}T_{e}}{m_{e}}\frac{n_{e}}{v_{e}}}} & (13)\end{matrix}$and K₁, K₂ are kinetic correction factors typically in the range of1.16≦K_(i)<2.8 and 0.3≦K₂≦1.5. When the ionization fraction is zero, themodel gives the usual values of thermal conductivity for neutral Argon.

From equations (10)-(13) the primary factors that affect the gain inthermal conductivity λ are the density of the electrons n_(e), and theirtemperature T_(e). For midrange and large χλ_(e) starts to dominate overthe other terms in (10). Based on this a gain is expected in the overallgas thermal conductivity when electrons are supplied at an elevatedtemperature via a nonequilibrium glow or corona discharge.

In this example, a stainless steel disk, block A, is heated using a 1 kWCalrod element. A large stainless steel cylinder, block B, is located ata distance of 4″ from block A and functions primarily as an electrode,and secondarily as a heat sink. FIG. 49 shows a schematic diagram of theexperimental setup, and FIG. 50 shows the dimensions of the blocks. Forthe temperature decay process, measurements were taken in presetpressure stages. Measurements were repeated using various inert gases asdescribed.

This example reports the experimental results of heat transfer gain innonequilibrium plasmas. In this experiment, an ORAM high voltage powersupply, Model number DSR 100-100-JTTF input 240VAC single phase 50 H2,output: 0 to −200 kV rectified provided the nonequilibrium plasmabetween the blocks. Throughout all experiments, the Calrod elementremained attached to the smaller block A, while the larger block Bcontinued to absorb a portion of the heat conducted from block A.Generally, block A was connected to the high voltage negative terminaland block B was grounded, although a few experiments were performed toexamine the effect of polarity reversal on heat decay.

In order to obtain comprehensive information for study, severalvariables were introduced into the experimental plan. These included:

-   a) different gases (air, Ar, He);-   b) different vacuum pressures (atmospheric, 10⁻¹ and 10⁻² Torr);-   c) different plasma currents (up to 25 mA and/or voltages up to 50    kV);-   d) different spacings between the block faces (1½″ and 4″).

However, for comparison of data, attempts were made to ensure that twoparameters were held reasonably constant:

-   a) the initial temperature of block A at the start of the decay    process;-   b) the duration of the heat decay measurement process.

The equipment available presented the following limitations upon theexperimental procedure:

-   i) A non-intrusive instrument for continuously measuring the    temperature of the high-voltage block from outside the cathode was    not available. An infrared optical pyrometer obtained for this    purpose proved unsatisfactory, hence the necessity of using    thermocouples.-   ii) Even though the voltage control was set low, the high voltage    source could unexpectedly generate a short pulse up to 60 kV when    first switched on. This was due to the type of circuit used within    the control unit, and to the inherent cyclical nature of the high    voltage generator.

The following measuring procedure was therefore adopted:

-   a) Thermocouples were successfully employed to measure the    temperatures of the two blocks. However, the temperature of the high    voltage block cathode could not be monitored continuously. Instead,    the high voltage was switched off at five minute intervals during    which temperatures were recorded from digital multimeters with    thermocouple module attachments.-   b) The meter arrangement was then disconnected from access terminals    protruding from the side of the vacuum chamber, and the high voltage    was again switched on. This measuring procedure meant that the high    voltage was disconnected once for a period of approximately 35 to 40    seconds during each five minute interval.-   c) The Calrod heating element remained disconnected during all heat    decay measurements. Therefore, it presented no obstacle to high    voltage plasma production.

A complete experimental procedure in air would consist of the followingstages:

-   1. Pump down vacuum chamber to approximately 10⁻⁴ Torr.-   2. Switch on Calrod heater for block A while pump cycle continues.-   3. Switch off heater at close to required initial temperature for    start of decay measurements.-   4. Continue pumping down from vacuum chamber to desired pressure.-   5. Repeat steps 2 through 4 until desired pressure and initial    temperature are reached.-   6. Switch off heater and immediately record the initial t=0 details    of block A temperature, block B temperature, vacuum chamber's top    skin temperature, and vacuum chamber pressure.-   7. Disconnect high voltage block (cathode) thermocouple meter    arrangement.-   8. Cover exposed terminal with an insulator cap.-   9. Switch on high voltage controller and smoothly bring it to the    required voltage or current.-   10. Each minute thereafter, monitoring grounded block temperature,    vacuum chamber top skin temperature, vacuum pressure, applied    voltage (in kV) and current (mA) displayed on the high voltage    control panel.-   11. At five minute intervals, switch off the high voltage supply,    remove the insulating cap, briefly ground all terminals exposed from    the cap area and connect the thermocouple meter arrangement in the    manner described in a) through c) above.-   12. Continue for an elapsed time of 131 minutes.

For experiments in inert gas, steps 4 and 5 would be modified slightlyto ensure the chamber was filled with gas, and that pressures higherthan required were not attained.

In this example, the following observations can be made:

-   1. In order to test all instrumentation, the initial plasma tests    were carried out in air at atmospheric pressure. This served several    purposes;-   a) the vacuum chamber door could be left open for observation of    plasma or arcing.-   b) the high voltage equipment could be used without the background    noise of the vacuum pumps, etc., for detection of voltage breakdown.-   c) the lighting could be dimmed for better observation of the    experimental area.-   2. Plasma was first witnessed when the gap between the block faces    was “1½” and the voltage was in the 41 kV to 47 kV range. The plasma    was poorly visible, being blue-green in colour, and was localized    and faint. Attempts to increase the current to a steady flow above    imA to achieve more brightness resulted in arcing problems and    voltage breakdown. 51 kV was found to be the maximum voltage that    could be applied for steady experimental results, but current flow    was less than 0.5 mA, i.e., lower than the multimeter's detection    threshold.

However, plasma was being produced and was accompanied by a sizzling“boiling oil” sound which began at approximately 27 kV. FIG. 51demonstrates the results when 51 kV was applied in air at atmosphericpressure.

-   3. Attempts to place two point sources (spikes) in block B (grounded    block) in an effort to improve the visible plasma volume were    ineffective and were not continued. Plasma was created at the point    tips from 34 kV, being a faint blue-green colour. The two spikes    were removed after these initial tests and not used again.-   4. The first test in partial vacuum was conducted with all other    conditions same as in 2 above. A CCD camera was fixed to the window    of the vacuum chamber so that the plasma could be monitored    throughout the experiment, and video recorded when desired. The    chamber was pumped down and kept at a steady pressure of 10⁻¹ Torr.    At this pressure, extensive plasma was easily created with voltages    less than 1 kV. A steady purple color was visible and an initial    experiment was conducted with approximately 20 mA current. FIG. 52    demonstrates the results achieved during this trial. In order to    investigate how temperature decay changes when the polarity of the    blocks are reversed, the wiring of the block was changed. Block A    was grounded and the block B was connected to the negative high    voltage terminal. All other conditions remained the same, except    that the thermocouples were interchanged for safety reasons. The    experimental run was then repeated. The plasma that was formed was    extensively purple. FIG. 53 compares the temperature decay data when    block A acted as cathode, against the result when polarity was    reversed and block B acted as cathode.-   5. For the remainder of the experimental study, the heated block A    was kept at negative potential while the passive block B remained    grounded.-   6. The gap between the block faces was next increased to 4″, and the    resulting effect was examined. This proved to be a more convenient    gap to work with because the larger spacing meant that:-   a) greater plasma currents, hence greater electron densities, could    be achieved without arcing.-   b) the plasma region was more visible through the viewing window.    This became more important at lower pressures, when the optically    bright part of the plasma occupied only a portion of the space    between the blocks.

The majority of the remaining tests were therefore carried out with the4″ gap. By reasoning similar to the above, a larger gap, e.g., 8″ mayhave provided even better results, but internal constraints within thevacuum chamber prevented this. Given the limitations of the equipmentand geometric constraints, the 4″ gap proved most practical. FIGS. 54-56show the temperature decay curves in plasma air, Argon, and Helium usingthe 4″ gap, plotted along with the decay curves.

-   7. Since the plasma heat decay in air at 10⁻¹ Torr demonstrated    results comparable with the best of the other gas media tested, air    was chosed as the medium to examine the effect of varying current on    the heat decay process. Tests were conducted at 10, 15, 20, and 25    mA plasma currents. Unfortunately, limitations of the high voltage    power supply controller resulted in frequent arcing and voltage    breakdowns, affecting data acquisition process. As a result, some of    the experimental trials were not run to completion.-   8. Feasibility tests in air at pressures less the 10⁻¹ Torr (say    10⁻², 10⁻³ Torr) suggest that higher voltages are required to    produce similar plasma currents. The low pressure tests also    demonstrated that the plasma structure within the block gap    resembled the classical discharge model, with glow column and    Faraday dark space becoming more apparent as pressure was decreased.    Due to the limitations of the equipment, maintaining a constant    current at the lower pressures proved to be more difficult, and the    arching problem became more pronounced.

FIG. 57 shows a graph of the data collected for air at 10⁻² Torr.Although arcing was again a persistent problem, and the data collectionrun was cut short to 40 minutes, it does provide further evidence of thetrend of increased heat transfer with increased current, even at theselow pressures.

The experimental results are summarized in the graphs FIG. 51 to FIG.57. The following is a discussion of some of the features of thesegraphs.

A heat transfer coefficient, γ_(x), is first introduced. Consider a timedependent heat transfer problem where a metal block is giving up heatthrough a thermally conductive neutral gas medium the heat decay curvewill be determined by the thermal diffusivity.$a = {\frac{\lambda}{C_{p}}.}$

Where C is the heat capacity of the medium at constant pressure, λ isthe thermal conductivity, and ρ is the density of the medium. Nowconsider a different gas medium, with conductivity λ, heat capacity C¹,and density ρ¹. In this case we have$a^{\prime} = \frac{\lambda^{\prime}}{C^{\prime}\rho^{\prime}}$

The heat transfer gain coefficient between these two systems can bedefined as $\gamma = {\frac{n^{\prime}}{\chi}.}$

Now, consider the case of neutral gas vs. plasma. For the case of plasmawith small ionization fraction X<0.1, the presence of electrons hardlyaffects the density of the gas, soμ≈μ

Now consider heat capacity. The molar heat capacity of electrons isextremely small when calculated via Fermi Dixac statistic. So thecontribution of the electrons to C_(p) can be ignored as a firstapproximation. Likewise the contribution of the ionic heat capacity canalso be ignored, again owing to the small ionization fraction.Therefore,C′≈C.

The gain coefficient becomes$\gamma = {\frac{\lambda^{\prime}}{\lambda}.}$

Since all other system parameters are the same between the two systems(pressure, metal composition, radiative heat flux, etc.), there reallyis no other mechanism by which heat decay time can increase or decreaseother than by gain change γ>1 or <1 between the two systems. In otherwords >1 implies an increase in the thermal conductivity of the gasmedium. The gain coefficient is a convenient way of comparing systemswith exactly same geometries and similar gas chemistry, the onlydifference being the introduction of nonequilibrium ionization. If γ>1between two systems then there is a gain in thermal conductivity due tononequilibrium ionization. If <1 then there is a net decrease in thermalconductivity due to nonequilibrium effects, which is an interesting andpossibly useful phenomenon in its own right.

-   1. FIGS. 51, 52, 54, 55, 57 all clearly exhibit γ>1, and    accordingly, the nonequilibrium plasma effect to have been clearly    demonstrated. The effect is most obvious in FIGS. 52, 54 and 55.-   2. FIG. 56, which is a graph of the data for Helium at 10⁻¹ Torr,    shows the mechanism of nonequilibrium plasma conductivity gain    discussed above. Consider the data presented in FIG. 55 for Argon,    and FIG. 57 for Helium, both of which are under comparable pressures    and applied power. Ignoring the ionic contribution to k as    negligible, the gains for these data sets can be written    respectively as    $\gamma_{6} = \frac{\lambda_{Ar} + \lambda_{e}}{\lambda_{Ar}}$ and    $\gamma_{7} = {\frac{\lambda_{He} + \lambda_{e}}{\lambda_{He}}.}$

The clear gain visible in FIG. 55 suggests that λ_(e) is not negligiblefor the Argon case. Now, the first ionization potential for Argon is≈15.3 electron volts, but for Helium, it is 35 eV. When considering thationization kinetics are exponentially dependent on ionization potential,it is entirely reasonable to assume that applied power to Argon willproduce more ionization than the same power applied to Helium undersimilar conditions. Therefore λ_(e) for Helium should be less than λ_(e)for Argon. Also, λ_(He) is an order of magnitude larger than λ_(Ar) tobegin with, so the combined effects giveγ<<γ₆which is clearly the case when the data in FIG. 56 is compared to FIG.55.

The practical implications of this are as follows. If it is necessary touse Helium as the working medium, the power supply should be modified tocompensate for Helium's high ionization potential.

3. Concerning FIG. 53. Referring to FIG. 57, it is clear that the gain γis actually a function of plasma current, i.e., γ=γ(I). That γ increaseswith current is not surprising, since higher currents lead to largerprobabilities of collisional and cascade ionization, finally leading tohigher values of electron density n_(e) and ionization fraction χ. Whatis surprising is the reversal of γ i.e., γ<1 when the polarity of blocksA and B are reversed, as can be seen in FIG. 53.

It now remains to assign some numerical estimates to the actual valuesof γ. The objective is to provide a simple model that can provide curvessimilar to the data sets in FIGS. 51-57, from which γ values can beinferred.

When gas temperature and electron density are actually distributedunevenly throughout a gas/plasma medium, the terminal conductivities ofthe gas λ and plasma λ are actually functions of x, y, z, t. Furthermoreλ¹ depends on the electron density n_(e) and the electron temperatureT_(e) both of which are functions of x, y, z, t, applied electric field,and current. So it is unrealistic to attempt to solve for n_(e) andhence λ¹ directly. However, it is possible solve for constant effectiveλ, λ¹ which model the net transport effect of the neutral gas andplasma. The resulting gain factor γ=λ¹/λ will be a good estimate of thetrue gain.

First, a model constructed by making a number of assumptions whichdrastically simplify the problem numerically without losing anyessential detail of the experimentally observed physical behavior.

a) Since the experimental setup is almost cylindrically symmetric, wemap the vacuum chamber onto a axisymmetric region in (z, r). Block A,block B, the insulator section, and the plasma region will be consideredaxisymmetric subregions of the vacuum chamber region.

b) The plasma, when present, is confined to a region surrounding bothblock A and block B. The extents of the plasma are chosen to be ratherlarge (not just confined to the gap between blocks A and B). Choosing alarge plasma volume a priori is a conservative policy, because if theplasma is measured by probes and found to be in fact smaller, then the γcalculated with the larger plasma volume will be an underestimate of thetrue γ.

c) For any given γ calculation problem, all thermophysical propertiesare considered constant.

d) Radiation reflected from the inner gap face of block B is ignored.

e) The Calrod element is eliminated and approximated by a thermal powerdensity applied to block A.

f) Radiative losses of the insulator and block A are approximated byequivalent thermal power density losses within each their respectiveregions.

g) The boundary of the vacuum chamber is convectively coupled to theexternal ambient environment. Actually, this boundary condition can befixed to some temperature (310K, for instance) without much differenceto the overall model. Convective coupling is handy in order to model theskin temperature of the vacuum chamber.

h) Radiation of block B is ignored entirely. Since block B's temperaturerange is typically 300K≦u_(B)≦450K, the error introduced by thisassumption is small.

It is desired to find the solution of the initial-boundary value problemfor the time-dependent heat equation.${{\nabla{- \left( {\lambda{\nabla{u\left( {z,r,t} \right)}}} \right)}} - S + S_{R} + {C_{R}\frac{\partial\quad}{\partial t}{u\left( {x,r,t} \right)}}} = 0$

Where C is heat capacity at constant pressure, ρ is density, S is theapplied power density source term, and S_(R) is the power density lossdue to radiation. The time dependent problem begins when the appliedpower is switched off, i.e., S=0 for a pure temperature decay problem.

The initial value is the initial temperature distribution at t−0₁u(z, r, 0)=u _(o)(z, r).obtained by solving the steady state problem∇·(−λ∇u(z,r))−S+R _(R) =O.with applied thermal power density source S and radiative power densityloss S_(R).

Each region has its own thermophysical properties λ, ρ, C, S, and S_(R).Radiative boundary conditions are treated in the following manner. Thetotal power loss P_(R) for a region due to radiation isP _(R) =A _(R)ε(u ⁴ −u ⁴ ₁),where A_(R) is the exposed radiating surface area of the region is theregion's emissivity, and u₁ is approximated by a constant referencetemperature in the range 300K-50K. If V_(R) is the volume of theradiating region, the radiative power loss density with the regionbecomes$S_{R} = {\frac{P_{R}}{V_{R}} = {\frac{A_{R}{ɛ\left( {u^{4} - u_{1}^{4}} \right)}}{V_{R}}.}}$

Experimental data sets like those in FIG. 53 consist of a controltemperature decay curve, where there is no plasma, and one or more decaycurves with plasma present. By fitting the time dependence of U(z, r, t)to curve sets like FIG. 53, we can directly compute estimates for γ canbe directly computed.

The above initial-boundary value problem would be straightforward if itwere not for the following difficulties:

1. Exact values for the thermophysical properties are not known a prioribecause the temperature distribution in the vacuum chamber is notuniform. Furthermore, there are slight pressure variations in theexperimental data due to the changes in temperature as a function oftime. The exact values for the emissivities of the materials under theconditions of the experiment are also unknown.

2. Simplification of the geometry, as discussed earlier, has the sideeffect of modifying some material properties in ways that cannot beknown beforehand.

3. Ideally it is desired to establish realistic bounds for many problemparameters and allow them to vary within these bounds.

The following, however, are known: theoretically, for a given data set(like FIG. 53) there should be only one parameter upon which thedifference in shape between control and plasma decay curves depend:=λ¹/λ. In other words, there should be only one unknown parameter. Inlight of this, a procedure for obtaining this type of parameterestimation is described. The technique can constructed for generalcases, where IBV models must be fit to curve sets that have dependencieson N unknown parameters. Description is provided where N=1.

Let φ₁, φ₂, . . . , φ_(k) be all parameters which define the model, inno particular order. These include geometric dimensions of the primaryregion and all subregions, all coefficients appearing in the partialdifferential equations, and all parameters appearing in the boundaryconditions. Suppose that M curves to which curve-fit parameters underthe hypothesis that these curve fits differ from each other by changesin one parameter. For the sake of simplicity, let us consider the casein FIG. 53 where M=2, i.e., there is an experimentally determinedcontrol curve f, and an experimentally determined plasma curve f′.Suppose some set of numbers

-   -   {φ₁, φ₂, . . . , φ_(k)}        results in a tolerable curve fit for f. Consider another set of        numerical data    -   {φ′₁, φ′₂, . . . , φ′_(k)}        that gives a tolerable curve fit for f′. Now, consider the        variations ô{acute over (ø)}_(j) between these parameters:        δφ_(j)=φ_(j)−φ′_(j)        For f and f′ to be related by at most one parameter, all these        variations must be zero, save for one:        δφ₁=0,δφ₂=0, . . . , δφ_(k−1)=0,δ_(K)≠0.        If the curve fits to f; f′ are good, and the IVB is well        defined, the resulting estimates for the uncertain fixed        parameters φ₁, φ₂ . . . , φ_(k) can be quite accurate, barring        pathological cases.

The following describes the application of this technique to the f, f′curves in FIG. 53. Some preliminary observations:

1. Consider all geometric data as fixed, and not subject to thevariation minimization procedure.

2. Express all thermophysical properties as φ_(j)x textbook values.Estimate gas density from equation of state and use this as a basevalue. Let textbook thermophysical properties and ideal gas densityestimates be superscripted by 0.

3. Define and organize all model parameters using a logical notation.Superscripts for φ are used as labels to identify what property theparameters affects. Subscripts indicate materials s=steel, i=insulator,g=gas (Argon), p=plasma.

4. It is possible to get an upper estimate on the maximum power densityapplied to block A. The Calrod heater is rated at 1000 W, and the volumeof block A is 4.633×10⁻⁴ m³, giving a maximum power density ofS=2.158×10⁶. But since this quantity is not exactly it can be replacedwith φ⁵. This is further constrained by the requirement that the initialmodel temperature be close to the initial experimental temperature inblock A.

Here is a complete summary of the model, and all parameters used andtheir initial estimates. All values are in MKS units.

Steady State Problem.∇·(−λ∇u(x, r))−φ^(s) S+φ ^(n) S _(R) =U,λ_(g)=φ_(g) ^(λ)λ_(g) ⁰,λ_(g) ⁰=0.01799  (14)λ₂−φ₃ ^(λ)λ₃ ⁰,λ₃ ⁰−23.43  (15)λ₁=φ_(i) ^(λλ) _(i) ⁰,λ_(i) ⁰=3.096  (16)ε_(s) ^(o)=0.08  (17)ε₁ ^(o)=0.18  (18)Time Dependent Problem. $\begin{matrix}{{{\nabla{\cdot \left( {{- \lambda}\quad{\nabla{u\left( {z,r,t} \right)}}} \right)}} + {\phi^{R}S_{R}} + {C_{P}\frac{\partial}{\partial t}{u\left( {z,r,t} \right)}}} = 0} & \quad \\{{{p_{a}C_{a}} = {\phi_{g}^{pc}p_{g}^{0}C_{g}^{0}}},{p_{g}^{0} = {1.784 \times 10^{- 4}}},{C_{g}^{0} = 518}} & (19) \\{{{p_{s}C_{s}} = {\phi_{s}^{pc}p_{s}^{0}C_{s}^{0}}},{p_{s}^{0} = 7758},{C_{s}^{0} = 431}} & (20) \\{{{p_{i}C_{i}} = {\phi_{i}^{pc}p_{i}^{0}C_{i}^{0}}},{p_{i}^{0} = 1595},{C_{i}^{0} = 753.1}} & (21) \\{\lambda_{p} = {\phi_{p}^{\lambda}\lambda_{g}}} & (22) \\{\gamma = {\begin{matrix}\lambda_{p} \\\lambda_{s}\end{matrix} - {\delta\quad o_{p}^{\lambda}i\quad 1.}}} & \quad\end{matrix}$

It is possible to retain $\begin{matrix}{{{{\nabla{\cdot \left( {{- \lambda}{\nabla{u\left( {x,r} \right)}}} \right)}} - {\phi^{S}S} + {\phi^{n}S_{R}}} = U},{\lambda_{g} = {\phi_{g}^{\lambda}\lambda_{g}^{0}}},{\lambda_{g}^{0} = 0.01799}} & (14) \\{{\lambda_{2} - {\phi_{3}^{\lambda}\lambda_{3}^{0}}},{\lambda_{3}^{0} - 23.43}} & (15) \\{{\lambda_{i} = {\phi_{i}^{\lambda}\lambda_{i}^{0}}},{\lambda_{i}^{0} = 3.096}} & (16) \\{ɛ_{s}^{0} = 0.08} & (17) \\{ɛ_{1}^{0} = 0.18} & (18)\end{matrix}$as the single parameter relating curves f and f′ while reducing thevariation of all other parameters to zero. This gives the estimate forconductivity gain.

The procedure outlined above is extremely time consuming to implement,so the estimation of γ is limited to the case of FIG. 53. The larger gapis preferable due to the smaller effect of reflected radiation. Afterconsideration iterations, the parameter set can be obtained: φ Curve fCurve f′ Variation δφ φ^(R) 1.0 1.0 0.0 φ^(S) 0.82 0.848 −0.028 φ_(g)^(oC) 5.0 3.0 0.0 φ_(i) ^(pO) 1.8 1.8 0.0 φ_(g) ^(λ) 1.0 1.0 0.0 φ_(p)^(λ) 1.0 10.5 0.5

All other parameters are simply unity, with zero variation. Theconductivity gain derived from the data set in FIG. 53 is thusγ₄=10.5.

Curve fitting results for f, f′ are displayed in FIGS. 58-59. Note thata small variation in Π⁵ was necessary in order to match the slightlydifferent experimental initial temperatures in curves f and f′. Whenconsidering that the plasma is extinguished for at least 10% of the timeduring the measurement time, the true plasma conductivity gain would besomewhat greater than 10.5.

Example 7

Calculating γ can be relied only on the temperature decay curves f(t)(no plasma) and f′¹ on block B by solving the 3D axisymmetric boundaryvalue problem. It can be assumed that block B is isothermal.

This example is set up by assuming that the plasma is large and notconfined to the interblock gap, ignoring radiation and convection, aswell as the insulator section, and using same dimensions for blocks andvacuum chamber as the example 6, and making the plasma region largerthan the gap. The temperature of the vacuum chamber boundary is fixed atT=300K For the exact dimensions used. Also, the following thermophysicalproperties are used (all units are MKS):

-   -   ^(p)metal=8000 ^(p)gas=1.0    -   ^(c)metal=431 ^(C)gas=518    -   ^(k)metal=29 ^(k)gas=0.018

These figures give us the thermal diffusivities^(α)metal=8.4×10^(−6α)gas=3.48×10⁻⁵which can be used in the time dependent problem. Subsequently, thesteady state problem is solved to obtain the initial temperaturedistribution T (z, r) and then plug this in as the initial value T (r,z, t=0) to a time-dependent boundary value problem. The time dependentproblem is solved twice. Once for γ=1 (no “plasma”), and then again forγ=10 (“plasma”). Here are the block A temperature decay curves for thisfictitious problem, as shown in FIG. 58:

Now, the lumped model for conduction isT ₂ =T _(s) T(T ₁ −T _(s))e ⁻³ t 2−t3where $\beta = \frac{aA}{V\quad\Delta\quad x}$

If all factors (other than k) remain the same between the control runand the plasma run, then the ratio of ζ's gives you the gain inconductivity: $\gamma = \frac{\varsigma^{\prime}}{\varsigma}$

This yields—0.94 for the argon data. In other words, the ratio of ζ'sobtained from the lumped model should give us roughly the same γ Istarted with. We can solve for ζ:$\beta = {{- \frac{1}{t_{2} - t_{2}}}{\log\left( \frac{- \left( {T_{2} - T_{S}} \right)}{{- T_{2}} - T_{s}} \right)}}$

As long as 300K<T₅<T₂, T₅ can be any value. From FIG. 60, we haveT₁(control)=570  (2)T₂(control)=329  (3)T₁(plasma)=570  (4)T₂(plasma)=304  (5)t=0  (6)t₂=600  (7)

Taking the following values for isothermal T₅ (again, any value can betaken, they are just to illustrate the issues):T_(s)(control)=320  (8)T_(s)(plasma)=303  (9)

The following values for are obtained:β=5.54×10⁻³β′=9.312×10⁻³and a conductivity gain of$\gamma = {\frac{\beta^{\prime}}{\beta} = {1.681.}}$

This is very much different from the value=10.0 upon which this sampleproblem is based. It may be worthwhile to check the Biot modulus Bi forthis problem.

Example 8

The example shows how colored water could be atomized successfully usinga high voltage DC source alone, and how experiments employing a eutecticalloy (wood's metal) looked promising. The apparatus used was to besubstantially refined before further liquid metal experiments continued.

An apparatus has been employed that has permitted limited atomization ofliquid metal when the flow is in an upward direction against gravity, orin a downward direction with gravity. A number of controlling parameterssuch as nozzle size, ambient conditions, spacing between electrodes,dielectric medium between electrodes, and shape and size of extractorelectrode have been varied such that an optimum environment foratomization of liquid metal to occur using only a DC source can beapproached.

The results are very encouraging and further substantial improvement insanticipated as the experimental apparatus is further refined, and when ahigh rise-rate AC signal is incorporated into the system.

-   -   i) Liquid metal flow against the direction of gravity, and    -   ii) Liquid metal flow in the direction of gravity.        i) Liquid Metal Flow Against the Direction of Gravity.

The apparatus used for these experiments is shown in FIG. 73. A copperreservoir is connected to the nozzle assembly by means of a copper tube.Solid pieces of eutectic alloy (wood's metal) are placed in thereservoir and heated to approximately 100° C. by radiation from two 150watt halogen bulbs. A pressure difference between the wider reservoirand the nozzle causes the liquid to flow to the tip of the nozzle, andthe flow rate from the nozzle can be regulated by the amount of alloyplaced in the reservoir. The reservoir and nozzle assembly is connecteddirectly to the high voltage terminal. A copper spheroid which is placedwithin {fraction (1/16)}^(th) of an inch from the nozzle tip surroundsthe replaceable nozzle, and a copper tube thermally connects thisspheroid to the reservoir assembly. In this way, not only heat isimparted to the eutectic alloy by conduction along the majority of itspath, but also a charge buildup area has been extended. The wholearrangement is painted black to behave as a black body absorber. A brassextractor ring is suspended at varying distances directly above thenozzle opening and is connected to a grounded electrical terminal bymeans of an adjustable mounting assembly.

ii) Liquid Metal Flow in the Direction of Gravity.

In this instance, a similarly copper reservoir is connected to a nozzleassembly by means of a short downward tube of thinner internal diameterthan the reservoir, and this arrangement is attached directly to thepositive high voltage terminal. An extractor ring sits at varyingdistances below, or to the side of the nozzle tip, and a collector cup,a partially filled with water sits for collection of atomized samples,directly below the nozzle.

A sensitive electronic balance was used to weigh drops and droplets fromatomization experiments. Control experiments were performed when dropswere produced by gravitational force and free hydrostatic pressure (fordownward liquid) alone. Repeat experiments were then conducted using asimilar head of molten metal, and a high voltage (max 36 keV, max 0.2mA) was applied to the fluid. Equal number of drops and droplets(nominally 100) were weighed, and their weights compared. FIGS. 71 and72 show the drops and droplets collected from one such series ofexperiments. For each figure the larger drops (upper portion of thefigure) are those collected during the control experiments, and thesmaller droplets (lower portion of the figure) are those collectedduring experiments using electrostatic field.

Under normal conditions, atomization of a liquid metal utilizing anelectric field between two electrodes alone is hampered by twoover-riding factors:

-   -   a) In air, at atmospheric pressure, arcing occurs at a voltage        lower than that required to atomize the liquid metal by        electrostatic means.    -   b) At pressures slightly less than atmospheric, there is an        inability to create potential differences between the electrodes        sufficiently high enough to enable atomization to occur because        the formation of plasma permits an easy path for current flow.

As a way of overcoming these voltage breakdown problems, a piece of CPVCpipe, ⅛^(th) of an inch thick was placed in such a way as to surroundthe extractor ring and its supporting arm (see FIGS. 74 to 76, and FIGS.65 to 69). By incorporating this CPVC into the assembly, substantiallyhigher potential differences between nozzle and extractor could beachieved before arcing occurred. This meant that more electrical energywas available for atomization, though the distance between theelectrodes could only be increased to a point where leaking would occurto neighboring components.

Liquid metal flow against the direction of gravity.

The nozzle aperture that permitted the best atomization was of the orderof 0.3 to 0.4 mm. For apertures smaller that this, difficulty wasencountered in securing a liquid metal flow. Apertures larger than thispermitted too great a surface area of liquid metal at the nozzle tipthan could be atomized by the experimental apparatus. Liquid metalatomization was captured on video camera, but could only continue for aslong as the head of pressure in the reservoir permitted flow of fluid tothe nozzle tip. A prelude to atomization can be witnessed as highvoltage is increased by either an increase in fluid flow rate, or as avertex (Taylor Cone) forming on the surface of the liquid metal at thetip of the nozzle. If too long a time elapsed between achieving acomplete flow of fluid from the reservoir to the nozzle tip andincreasing the high voltage from zero to a level sufficient to permitatomization, then satisfactory atomization would not occur. Instead,cooling and oxidation of the free liquid surface would be apparent andthe liquid metal would flow downward. The potential difference betweenthe nozzle and the grounded steel base plate was sufficient to increasethe liquid flow. Thus, a “pipe” of solidified metal would slowly formdown the side of the nozzle assembly until the pressure difference atthe reservoir decreased to zero.

In this manner, efforts to achieve atomization by means of a combinationof mechanical and electrical forces were successful. One such experimentinvolved placing a plunger over the open end of the reservoir toincrease the pressure difference at the nozzle tip. In this case,production of droplets by electrostatic atomization required lesspotential difference between the electrodes, and droplet sizes weresmaller.

In another experiment, the eutectic alloy was removed from the reservoirassembly and was replaced by water. The water was then drained from theassembly, leaving the inner walls wet. The eutectic alloy was thereturned to the reservoir and reheated. When liquid metal flow from thenozzle occurred, drops were ejected by the force of stream that wastrapped within the nozzle assembly. When a high voltage was applied, thedrops became atomized into smaller droplets, some of which adhered tothe undersided of the CPVC.

FIGS. 74, 75 and 76 show consecutive frames of the atomization of liquidmetal against gravity without any applied mechanical force other thanthat due to the head of liquid in the reservoir.

Liquid metal flow in the direction of gravity.

Liquid metal flow in this direction can be controlled much moresatisfactorily. Conditions can be created such that a continuous flow ofmolten metal is produced and nozzle apertures can be much smaller.Currently nozzles with apertures in the region of 0.1 to 0.15 mm arebeing employed and can be such that gravitational forces alone cannotovercome the adhesive forces of surface tension upon the inner walls ofthe nozzle, and the cohesive forces within the surface of the metal. Insuch instances, electrostatic atomization can produce droplets withdiameters of substantially smaller magnitude.

Rapid cooling during droplet flight provides this evidence. If theflight path were sufficiently long enough, and the environment in closeproximity to the apparatus could be maintained above the temperature ofthe liquid metal melting point then the droplets would achieve a morenearly spherical shape.

This is true for a particular stage of electrostatic atomization. At thetime of the Interim Report, we had reached a stage that we now dub“primary atomization”. Since October 28, we have had some partialsuccess in achieving secondary and (to a lesser extent) tertiaryatomization. Rapid cooling during flight continues to provide thisevidence.

Evident by experimental observation. Current nozzles being employed arestainless steel tubing with an outer diameter (O.D.) of 0.012 inch (0.3mm) and an inner diameter (I.D.) of 0.006 inch (0.15 mm).

Binary division occurs in some instances, but our ongoing studiessuggest that the production of smaller droplets may not be due simply tosuccessive binary division. This matter is discussed further herein.

Undoubtedly this is true. However, current research is aimed atdiscovering whom, and in what manner smaller droplets are produced.

Until we can employ a high resolution camera utilizing high speed film,we have no further comment upon this phenomenon.

Example 9

Rapid cooling during droplet flight provides this evidence. If theflight path were sufficiently long enough, and the environment in closeproximity to the apparatus could be maintained above the temperature ofthe liquid metal melting point then the droplets would achieve a morenearly spherical shape.

This is true for a particular stage of electrostatic atomization. At thetime of the Interim Report, we had reached a stage that we now dub“primary atomization”. Since October 28, we have had some partialsuccess in achieving secondary and (to a lesser extent) tertiaryatomization. Rapid cooling during flight continues to provide thisevidence.

-   -   were all evident to the client when a demonstration experiment        was provided for him on the occasion of his visit on December        14.

We have no further comment at this time.

We now discuss section E. Conclusions:

Evident by experimental observation. Current nozzles being employed arestainless steel tubing with an outer diameter (O.D.) of 0.012 inch (0.3mm) and an inner diameter (I.D.) of 0.006 inch (0.15 mm).

Binary division occurs in some instances, but our ongoing studiessuggest that the production of smaller droplets may not be due simply tosuccessive binary division. This matter is discussed further herein.

Undoubtedly this is true. However, current research is aimed atdiscovering whom, and in what manner smaller droplets are produced.

Until we can employ a high resolution camera utilizing high speed film,we have no further comment upon this phenomenon.

Smaller gauge stainless steel tubing has now arrived, as has a eutecticalloy containing Indium that has a melting point of only 47° C., and,delivery of high voltage control units (currently our units have amaximum range of 50 keV, Bertan model 815-30P) supplying up to 100 keVDC is imminent.

The means by which atomization occurs is by no means clearly understoodat this time. However, we continue to review our thinking as each newpiece of evidence becomes available. In deed, in recent days we havefound it necessary to review our thinking yet again in the light of newexperimental information. The extractor ring seems to play a significantrole in maximizing the division process. The most appropriate materialfor its composition, and its best location within the electrostaticfield remain uncertain at this stage. Ongoing experiments revolve aroundusing i) bare copper rings, each with a different radius, and ii) ringscomposed of a variety of good dielectric materials such as PVC.

The wire rings seem to permit some expansive (sucking) force to beapplied upon the droplet as it passes the ring plane, whilst the PVCrings seem to permit a compressive force to be applied upon thedroplets. The importance of maintaining the environment in the vicinityof the electrostatic field at a temperature above the melting point ofthe liquid metal cannot be over-emphasized. As the droplets becomesmaller their surface area to mass ratio increases and they cool morerapidly.

-   -   i) A positive high voltage DC source connected to the liquid        metal reservoir produces an electric field between the nozzle        and the grounded collector cup. The force of the field produced,        acting together with gravity, causes atomized droplets of        similar size to be collected. We have called this phenomenon        primary atomization.    -   ii) The placement of an extractor ring between the nozzle and        collector cup and concentric to the droplet path causes lateral        forces to be applied to the droplet, which can produce        successively small droplets. We have called this phenomenon        secondary and tertiary atomization.

A deeper understanding of the atomization process will enable us toimprove these numbers. FIG. 61 shows the weight of the droplets producedvs the gap between the nozzle and the extractor. This fig. clearlyillustrates that once a critical value is reached, primary atomizationis not sensitive to the high voltage potential applied, or to thedistance between the nozzle and the extractor. Table 1 and Table 2 inFIGS. 63 and 64, respectively show the results of experiments using barecopper wire extractors with different ring diameters.

FIG. 62 shows evidence abstracted from the same experimental sample inan attempt to understand the amortization phenomenon. All droplets wereproduced by the same experiment. The choice of solidified droplets P₀ toP₄ is an attempt to demonstrate the way in which the primary dropletsare subdivided. The choice of solidified droplets S₀ to S₄ is an attemptto show this phenomenon then repeats upon the secondary droplets toproduce tertiary droplets such as solidified droplet T₀. Droplets P₄ andS₄ appear severed, but they are not. They are whole; and droplet T₀seems large for its weight. Unfortunately, these apparent anomaliesarise from lens distortion due to the scanning and copy processesinvolved in producing FIG. 62.

Each patent application and publication referenced herein is herebyincorporated by reference herein in its entirety.

Various modifications of the invention, in addition to those describedherein, will be apparent to one skilled in the art from the foregoingdescription. Such modifications are also intended to fall within thescope of the appended claims.

1. A method for atomizing a particle comprising: producing a firstmolten particle; applying a rapid electrostatic charge to the firstmolten particle, wherein the rapid electrostatic charge causes the firstmolten particle to form at least one smaller second molten particle; andcooling the second molten particle by producing a non-equilibrium plasmathat transfers heat from the second molten particle to a first heatsink, wherein the first heat sink is electrically charged or held at apotential.
 2. The method of claim 1, wherein the first molten particleis produced by melting a material in a melt chamber, and expelling thefirst molten particle through at least one orifice in the melt chamber.3. The method of claim 1, wherein the rapid electrostatic charge is anarc discharge or an electron beam.
 4. The method of claim 1, wherein thenon-equilibrium plasma is a glow discharge or a cold corona discharge.5. An apparatus for transferring heat between a first heat transferdevice and a workpiece comprising: the first heat transfer device,wherein the first heat transfer device is electrically charged or heldat a potential, and wherein the first heat transfer device comprises aheat sink or a heat source; the workpiece, wherein the workpiece ismechanically separate from the first heat transfer device; and a meansfor transferring heat between the workpiece and the first heat transferdevice comprising a means for generating a non-equilibrium plasma. 6.The apparatus of claim 5, wherein the non-equilibrium plasma is a glowdischarge or a cold corona discharge.
 7. The apparatus of claim 5,further comprising an external means for generating or maintaining thenon-equilibrium plasma.
 8. The apparatus of claim 7, wherein theexternal means for generating or maintaining the non-equilibrium plasmais a thermionic emission, an RF electromagnetic radiation, anelectromagnetic radiation, a magnetic field or an electron beam.
 9. Theapparatus of claim 5, wherein the first heat transfer device comprises aplurality of heat-transfer devices.
 10. The apparatus of claim 5,further comprising a second heat-transfer device that is mechanicallyand electrically separate from the first heat-transfer device, whereinthe second heat-transfer device comprises a heat sink or a heat source,and wherein the potential between the fist heat-transfer device and thesecond heat-transfer device produces a non-equilibrium plasma.
 11. Amethod for transferring heat between a first heat-transfer device and aworkpiece comprising producing a non-equilibrium plasma that transfersheat between the first heat-transfer device and the workpiece, whereinthe first heat-transfer device is electrically charged or held at apotential, wherein the first heat-transfer device is mechanicallyseparate from the workpiece, and wherein the first heat-transfer devicecomprises a heat sink or a heat source.
 12. The method of claim 11,wherein the non-equilibrium plasma comprises a glow discharge or a coldcorona discharge.
 13. The method of claim 11, further comprising anexternal means for producing or maintaining the non-equilibrium plasma.14. The method of claim 11, wherein the external means for producing ormaintaining the non-equilibrium plasma comprises a thermionic emission,an RF electromagnetic radiation, an electromagnetic radiation, amagnetic field or an electron beam.
 15. A preform produced by the methodof one of claims 11-14.
 16. The preform of claim 15 which is a near netpreform.
 17. An article of manufacture produced by the method of one ofclaims 11-14.
 18. The article of claim 17 which is a component of a gasturbine engine.